Not often. Probably once a month on average. I don't dry clean. I know some folks say you can do it without scratching but you can't prevent swirls with no rinse cleaning systems despite claims.
I used to use 3 buckets...one for soapy water and 2 for first and second stage rinses. But I stopped doing that about a year ago when I instead bought 14 micro fiber wash mits and use just one bucket with about a gallon of water with soap. There's no danger of not properly rinsing a mitt because each mitt is used just once(one panel or part of a panel per mitt side) and then the mitt goes in the dirty bucket. Never rinse the mitt and since I don't re-use the mitt, only one gallon of water is needed plus no rinse buckets.
That said, I use the most water in the pre-wash and rinse stage. First, I power wash the car to blast 90% of everything off. That's about 2 gallons. Then I use the power washer with the soap dispenser nozzle to coat the entire car in car wash. That's about 1/2 gallon. Then another 2 gallons to power wash the soap off. Then I wash with the mitts. Then I power rinse with about 2 gallons. So in all, I'm using about 7.5 gallons of water.