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.