I make 10 gallon batches. I usually start with 12 gallons for a 60 minute boil, and 13 gallons for a 90 minute boil. I boil off about 1.5 gallons per hour, but I leave some trub in the BK and I have some losses to my CFC and tubing.
I use fermcap in the boil to help prevent boilovers.
Broken Leg Brewery
Giving beer a leg to stand on since 2006