Say a Bible since those were fairly common and a relatively consistent length.

You'd think that, but the Bible as the unit we currently think of today is actually a relatively recent invention coming from the thirteenth century. Plus, after the twelfth century they usually had fairly extensive glosses.

Copying books in an earlier medieval (say pre-1100 for convenience) monastic setting was a solemn and sacred task. Copying was a task only given to monks in their late twenties or older, as it required a certain placidness of mind that younger monks were not thought to possess. The scripts used in these books were usually large and stately, and are thus known as "book hands", usually with a moderate usage of abbreviations (the abbreviation system for Latin is extensive and regular, but beyond the scope of the question). Speed was not considered an issue, but only the perfection of the work. The Rule of St. Benedict mandates that a monk be given a book each year to read, and perhaps copy, so this would be a halfway decent guess. It was certainly considered a laborious process. We have quite a few scribal additions at the end of manuscripts, all of which I've misplaced right now, the paraphrase of one is "Finally that's finished, now give me some wine!"

With the rise of non-monastic learning from the late eleventh century onward, there was a greater need for books, and thus several techniques were incorporated. The most effective of these was a system which a student would take home a quire (usually four sheets folded over) from an unbound manuscript to copy. Unbound manuscripts are quite common - a binding could easily cost as much as the manuscript itself! - and thus a whole book might be copied in fairly rapid order by several different scribes at once, and the students would get a bit of money for running about town harassing honest citizens. The script used in these books, predominant from the early thirteenth century, was a more rapid cursive form of the Gothic book hand you might be familiar with and looked like this - and that's a very, very neat copy. As you might notice, this script is also highly abbreviated for speed.

Did illuminated texts take substantially more time to create?

Yes, absolutely. Rubrication and illumination were almost always done separately from and after the copying process. In the later period, master illuminators set up shop and would add pictures to finished works, particularly Books of Hours, which are sort of early versions of the Book of Common Prayer.

So, sorry that's not a very precise answer, but unfortunately no one really wrote down how long it took them to do any of this!