These charts to follow may also help. Also if you post your specific plane and set-up then perhaps someone with a similar set-up will be able to give you an estimate on flight times. The way I do an unknown plane is to take a wild conservative guess and research the manual or posts and then set my timer to that (usually 5 minutes)

I get used to the plane and trim it then measure the remaining capacity when I land. Then I can accurately calculate a new timer setting kinda figuring if I'm going to get more aggressive or how I may fly it in the future. It is important to do this again when making major changes too such as battery,motor,prop,ect.