This answer comes from the knowledge base of a musician, not an historian. I'm offering it as a supplement to whatever historical knowledge can be offered, as I believe that much of the question's premise rests on a lack of understanding when it comes to how an orchestral work is composed.

While different composers in different eras and times had varying access to musicians (Stravinsky, for example, could try out roughly anything he was working on with an orchestra during his schooling days) there are a few things that have held true throughout the history of Western music, which I'll address here.

The first is generally that orchestras are expensive - you, or your patron, are basically renting a room full of people (typically 24-48, but sometimes going up into the hundreds) for several hours. There are also rarely situations even under the patron system of the common practice era where an entire orchestra is on call and working for a single employer full-time, so even if you have your own orchestra you can't necessarily call them in for whatever you want. You're most likely to have an orchestra at your disposal for whatever you want if you're in a conservatory situation, and there you will still probably have limited access to them, no matter the time period, as you are one of many student composers and they have other works to rehearse.

Clarifying what I meant by "common practice" above: note that while classical music typically covers multiple periods (renaissance, baroque, common practice, romantic, late romantic, etc.), when we say "classical" we usually imagine the music of the common practice era, stretching over the Baroque, Classical, and Romantic period - roughly 1600 to 1900.

Here now we must differentiate between the composition (rhythms, pitches, harmonies, and melodies), and the orchestration (which instruments are assigned to which roles). Put simply, the composition is the song, and the orchestration is the color of the song. One can remove the colors and put all the notes of an orchestral composition onto a four- or eight- handed piano part, as most studying composers frequently do. (Google "Petrushka Piano" for good examples of how strongly a piano rendition can represent an orchestral work - Duo Curbelo's is one of the more fun ones, and demonstrates much of what I'm about to talk about.)

Until about the 20th century, when orchestration became its own kind of composition (with Ravel's Bolero, and several works by Stravinsky, Holst, and Schoenberg exploring the possibilities of orchestral color and texture), the melody and harmony was the most important element of an orchestral piece. What this means is that a composer can get very far in the composition of an orchestral piece without ever even touching on the fact that at some point, an orchestra will be playing it. This is analogous to the way that a modern songwriter might write an entire song on the piano or guitar, and then flesh it out into 140 instruments in a considerably smaller amount of time.

When writing a symphony in this style, you're usually dealing elements that serve either the melodic ("lead") parts, or the accompanimental ("background") parts. Before the orchestration emerges, there's usually a very strong sketch of melodies, harmonies, and textures that can be dummied up on the piano. While a composer like Mozart gave us a lot of works that just flowed straight from his brain to the page and to the orchestra, more laborious composers like Beethoven give us loads of early sketches for works, often with no accompanying rehearsals on record - so they must have had some way of testing out these pieces.

In fact one of the primary advantages of the piano is its large range of notes - while it cannot replicate the colors of the orchestra, it can replicate the range of notes and rhythms being played, excluding unpitched percussion parts and the longer held notes, which can usually be imagined or approximated easily enough when composing alone at the piano.

As it gains in complexity, a composer might need more hands than he has (they're almost always male in those days - you try getting an orchestra to play a woman's compositions in 1790). The accompaniment part has grown to encompass much of the bass and midrange of the piano and probably includes its own "descant" lines (melodic lines other than the main lines), while the melodic part likely has octave and third doublings and other things that have thickened it beyond what a single hand can play. At this point the composer will probably have a four-hand sketch of the piece that he can test out using a second pianist. This will give a fairly strong idea of what's working and what's not.

Once the piece is composed, it's time for orchestration. Oftentimes, the composer will have been orchestrating WHILE composing, but in other cases - particularly with Liszt, who often hired an outside orchestrator - the orchestration was a paint-by-numbers sort of deal that could be done just by looking at the four-hand piano part or sketch and knowing from experience and study what parts and figures would be best served by which instruments. Each instrument has a range of notes, technical limitations, and coloristic nuances that are well-known to any composer with a decent amount of study and/or experience, so it's not likely that a composer will be making too many errors along the way. A composer might try out some novel colors and might take an instrument or section out of its comfort zone, but usually not by much, and not in such a way that is irreversible should they find that it doesn't work in the rehearsal room.

So, as we demonstrated, the composition can be done entirely in the head or at the piano (or pianos, as it were), while the orchestration amounts to an assigning of notes with some creative embellishments according to well-known guidelines. This means that the first read with the orchestra - which need not be long before the premiere - will typically be for the conductor and players to familiarize themselves with the piece, and for the composer to 1) have an opportunity to clarify and tweak small things ("Maybe I should have the strings playing a tremolo instead of a trill there"), 2) see if a few bold experiments worked ("Hey, I guess those two sections blend better than I thought!"), and 3) listen for copyist errors ("Shit! That should have been an A sharp!"). If your composer is using the orchestra to find out if the symphony has the right chords and melodies and is the right length, you're in trouble, and there isn't much record of that sort of thing happening. The orchestra shows up when the composer pretty much knows that the piece is done - or, barring that, the composer was on a deadline and won't have time to fix things anyway.

All this said, a composer may also meet with various soloists (or friends who play the soloists' instruments) during the process to try out individual lines, especially if they're composing a concerto, which is a piece that is typically meant to showcase a single performer along with the orchestra, and which often stretches them and the capabilities of their instrument. But this is much, much easier to organize ("Hey Franz, come over for the evening and play some stuff, I'll get you drunk") than a full orchestral reading ("Hey, forty-eight skilled people, come over and read this part, I'll get you drunk").

Perhaps a good example of this comes from - of all places - the American Musical Theatre, where songs are almost always written and orchestrated by two different teams or individuals. Until the invention of synthesizers and MIDI sequencers which allowed composers to dummy parts, an orchestrator had no way of really knowing what the orchestra would sound like until a week or even a few days before the audience started coming in for preview performances. They'd write out parts for five weeks using a combination of paint-by-numbers, educated guesses, and a few creative and informed stabs at newness, and then an orchestra would show up and play it and they'd have a few days - maybe less - to frantically rewrite and fix. Film music is roughly the same - in fact, for a film they might have had a single day with the orchestra and no rehearsal.

In short: they don't have to imagine the sounds of the notes and the rhythms - they can do this with a piano or other chordal instrument. They DO have to imagine the sounds of the orchestration, but that's an easier matter, as the orchestral colors are typically very distinct, particularly as they were used in the common practice and early romantic eras. If you have a fast, high line, it's probably going to a flute. If you have a dark, lyrical line, it's probably going to an English horn, or a blend of low horns and strings. Most of your chordal and rhythmic accompaniment will be on the strings. Got some fanfare-like leaps or rhythmic stabs? That's your brass. Got a rich high-midrange chord? Woodwinds, in what are basically three accepted groupings to choose from. Percussion's even easier - in this era, the abilities of the percussion section are greatly limited, so usually you're just getting some accenting and reinforcement from timpani, bass drum, snare, crash cymbals, and so on. If you've copied a few scores, done some studying, and gotten to assist on a few big projects, you can give it a shot on your own, get your teacher to help you with it, and have a pretty decent orchestration. In those days, at least. And if you didn't do well, so what? Your piece sucks, you don't get another chance with an orchestra, and no sources are preserved saying what a shitty orchestrator you were. Thankfully for the world's listeners (if not its music historians), many of the crappy symphonies have vanished forever.

A good analogy here might be painting. Paint is expensive, but charcoal is cheap(er). A painter might make several black-and-white sketches of different objects, piece them together into a larger work, and get a very strong idea of where each object is to be placed - before a single color is applied to a canvas. Meanwhile, the general colors would probably be pretty obvious ("this apple should be reddish!" as a stand-in for "these repeating 8th note chords should be french horn and violas and 2nd violins") though their subtleties might require some planning. In this analogy, the piano arrangement is the sketch, and the orchestration is the coloring.

For a source demonstrating the process of orchestration (albeit in the 20th century, and outside of the classical world), Steven Suskin's The Sound of Broadway Music: A Book of Orchestrators and Orchestrations is an excellent resource. I can use my google-fu to see what else I can unearth regarding classical orchestration if more sources are required, as (thankfully for my need to stay awake) most of my academic music history books have gone missing over the years.

tl;dr: Excepting students and the rarely privileged composers, most did not have tremendous amounts of rehearsal, and like orchestrators throughout time most didn't need too many orchestra read-throughs during the writing process, as the orchestration is only but a portion of the writing process.

Edit: added a tl;dr, clarified a few thingamabobs, added more information on the actual process of orchestration.

Edited again to fix some crappy use of parentheses.

Edited again because I screwed up my era categorization. Thanks to u/EmperorofMeow for the correction

One final edit, days after the fact: Another element that would make test-playings of an orchestration prohibitive would be the labor-intensiveness of parts preparation in those days. Copyists today, with loads of music notation software, transposition at the press of a button, and printers, still require quite a bit of advance time and compensation to get parts ready. Imagine spending a week or two hand-copying and proofing 48 instrumental parts, at great labor and expense, and then taking them all back for proof-reading. It's prohibitive for many reasons; you're better off handing parts out at a time when all changes can be made with a pencil/pen.