Ya don't feel too bad. i'm sure that there are TONS of people on here who smoke WAAY more than you do ((myself included ;p if i have a very relaxing day off, i could probably kill 3 bowls, about 1:15 duration a piece, in a sitting. Daily for sure, at least 1 bowl. usually 2 or 3))
And it's not bad that your friends only smoke for 20 minutes... but it's also not bad that you smoke the whole bowl.
actually... a downside to smoking only a half a bowl is that it is kind of a waste for shisha unless you have a itty bitty bowl.