How many times do you use a gallon of paint? How do you dispose of it once you're done with it?

depends... if it just paint and not to many bodily fluids (cum) or other messy items have been mixed in the you can use it indefinitely.

if its in conjunction with a food item i would keep it max a couple days to a week. just put it in a black garbage bag and trash it. or if you care about the environment take it to your towns recycling center/dump. they will separate hazardous waste

I've recycled paint over and over again for well over a year with no problems. I buy the trade 10l tubs as they're relatively cheap, last ages and are a good size for head dunking in.

I never used that much paint. I used wall paint /water based/ and wood paint /oil-solvent based/ and I just flush it down the drain, and I never had any piping problems.

Joint compound is different case - for that I remove bulk and toss it in garbage bags and wash remains with plenty of water, it caused few clogs but not serious.