The amount that you lose to boiloff is going to be dependant upon a number of factors - the vigor of your boil, the altitude of your location, the ambient temperature, the size of your kettle, etc. I'd say that Taipans is correct that 1g/h is a good rough estimate, but the only way to really find out is to do a full boil and measure your volumes carefully both before and after.
As for how much to start with, that depends on all of the factors above as well as how much you want to be able to lose in transfers. I always shoot for 6 gallons of wort at the end of my boil. That way I can leave 1/2 gallon of slop behind in the kettle, and I can leave as much as 1/4 gallon behind when I rack from primary to secondary and another 1/4 gallon when going from secondary to keg.
On my system I collect just shy of 7.5 gallons of wort and boil down to 6 gallons in 60 minutes. Your results will vary.