[DAVES]

Water use depends on, how much you water. When it is hot and dry, little rain, you need to water more to keep your grass green.
Obviously, a larger lawn will need more water to cover than a smaller one. Then run time to apply what you have decided is the needed amount of water depends on the heads you have and the water pressure you have.

You can not worry about all the above. Scatter cans around you lawn. Washed cat food, tuna type cans work great and you can easily measure how much water comes out in a known time as well as how evenly your lawn is covered. Once you know the problems, you can find and correct the issue. You could have blocked heads.
Missing heads-they are often hit by mowers or plants blocking the head pattern.

As to cost. We actually have at least two different systems in The Villages. In the north they are using drinking quality water for lawn irrigation. In the southern sections it is reclaimed water that is not purified enough to drink-a separate system al together.
On the reclaimed system those who use more water actually pay more per gallon. Sort of like a graduated tax. Your price per gallon starts at the least expensive rate and as you use up each block, it goes to the next higher rate per gallon. If, you call your water supplier surely they can explain it, better than I, tailored to your particular bill.