It all depends on how far back you go. I can speak well to the 16th century and onwards that it definitely moved much faster than today. The gentleman in 1780 would more likely say "that cuff is so 1778!". Fashion was changing 4-5 times a year and not just for the wealthy. If you need a new item of clothing, you're going to buy a fashionable one. It's not more expensive to have a cuff of a coat cut slightly narrower. Perfect example is this image from 1793 making fun of the fashion changes from 1778.

Having clothing re-fashioned to update it was also common. A slight alteration in trim or cut can make all of the difference. It was the fabric that was the investment. In fact, a plain ladies gown of this time takes about 10 hours to make (and I have verified this in practice numerous times). That's a days work, so it costs a days wages. Here in Virginia they are charging 2-3 shillings to have the labor done according to account books. Most fabrics are at least 2 shillings a yard (it takes three or more yards for a gown). Having the gown or suit cut to fit the latest fashions doesn't cost more, the job of the tailor or mantua maker is to keep up with the trends. Also, the only places that have second-hand clothing trade are the very large cities: those areas with a large under-employed and poor class. Second hand clothing shipments are turned away in better areas.

There's a perfect example in my area of study. Lady Mary Coke traveled to Brussels from England and fell in love with a regional style of painted leather ladies shoes in 1767. She purchased six pairs and brought them back with her, starting a short fad. We can see painted leather shoes of this style earlier in the century and in the 1790s, but any shoes of the 1760s style that are painted leather and have an English source can be pinpointed to that fad of only a year or two. Likewise I can date almost any shoe from the 18th century to a 10 year period with no difficulty. Some are easily within 5 years. People wear shoes out quickly and buy new ones a few times a year. The shape of the heel or the cut of the quarters doesn't affect price, so keeping up with style is easy.

Earlier, the evolution of the French Hood of the 16th century is a good example. While it was worn for about fifty years, the shape and style of the piece changed throughout. Subtle to us in modern day, but it would have been distinct to them. Jeans from 1988 are different than today, but the general style has still been worn for decades.

Yes, simply because:

A) few people could afford new clothes.

B) Most clothing was hand-made. If it fit and kept out the elements, then it really didnt matter how it looked.

However, style guidelines would start generally in a line beginning within royal courts, filtering down though lesser nobles, then to the "middle class" and maybe you would see styling cues in outfits among poorer people. Each step of the way you lost frivolous frilly accents that were simply too expensive for lower classes to replicate or just weren't useful. Style changes were "quicker" for the rich for sure, but not at any speed that we could appreciate, and the changes were unevenly spread.

Kinda sounds like what you have today. The difference now is that clothing is so easily mass-produced that virtually any style can be on the catwalk in October and then be in a store like H&M or Macy's by Thanksgiving, although, like back then, some of those expensive styles are paired-down and use cheaper material for places like Wal-Mart.

When you look at fashion trends starting around the early 19th century, each decade started showing more and more unique styling for that very reason. Styles became easier to replicate and quicker to produce (Industrial Revolution). Fashion fads could now, thanks to easier distribution (e.g. Sears Catalog), spread all over the world. Global advertising and fashion magazines (Harper's and Vogue) also replaced the crap-shoot relying on word-of-mouth to set tastes.

Today it is so fast that, you are correct to have noticed, it takes far longer for something to go out of style. Fashion is so fast that people simply cant keep up. Future historians will have a tough time really understanding what the dominant fashion trends of these early 21st century decades were as opposed to the brainlessly easy decades of the 20th century.

Retro style movements and the prospects of "3D printing" clothing at home will make it even harder.