There are an awful lot of variables so any rule of thumb is just a guideline. I have the same rule of thumb for pretty much all high end equipment, not just speakers. So here it is.
I expect to see depreciation in the 25%-33% range for equipment that is up to two years old. By two years old, it should be depreciated by a third. I then find that high end equipment will hold its value pretty well at roughly 50% from year 2 to year 5 when the warranty typically runs out. After that I expect another significant drop to about one third of its original price. Adjustments are made to these figures for physical condition.
That's a pretty gross generalization, but it is what I notice. For a particular brand or item, if you're talking about spending a lot of money, it would make sense to follow listings on the used market before you buy so you know what prices to expect. It might also be a good investment to check out the AudiogoN blue book.
I'll be interested to see what other posters think is a good rule of thumb.