The amount of gallons of water needed to water a lawn depends on the size of the lawn you're watering.
But for example if your lawn is a 10x10 area of lawn in size then it would take around 62 gallons of water per that area of lawn.
For a 20x20 lawn you'd use 124 gallons of water and for a 40 x 40 lawn you'd use 248 gallons of water so most lawns will use less than 1,000 gallons of water when watering the lawn.
If you run your sprinkler for 1 hour to water your lawn then you'd use around 1,000 gallons of water which isn't very expensive and helps keep the lawn looking nice and healthy.