Grog, I think your question got lost in this thread. You have probably done some Google searching on this topic already. I didn't know the answer but was curious myself so I did.
It depends on your equipment, what you are boiling, your environment, etc. to really answer that question. You have to do the math yourself the next time you brew. The general rule is around 9-10% evaporation rate per hour it appears.
How you determine your own is the next time you brew is to take the Starting volume measurement and divide it by the ending volume measurement and that is your evaporation rate.

So, if you are doing a full 5 gallon boil (which I don't do, just do the whole 2-3 gallon and top off method) boil that 6.3 or whatever and see what you have at the end. Your volume will be affected by what you add to the recipe etc.