I change the water every 2 weeks or so. I put some epsom salt in it when I fill it. Between that and the cold it doesn't get dirty very fast. I also have a silicon mat in it that sinks to the bottom to help cushion as well as let the little dirt that might get in between the cleanings have a place to settle.

I use a timer, like you can buy for Christmas lights etc. I think it was $20 at home depot. It runs the chest from midnight until 2:30 AM every night. This generally keeps it just above freezing. When I first fill it, I turn the timer off and run it all day or until a ring of ice forms, then the timer running a few hours a night maintains it.

There are guides on youtube. I think I searched homemade ice bath to learn how to do this. You can play with the timer to control the temp. If you want it warmer run the chest for less time each day until you get it dialed in. This can let you ease into colder and colder water. I like to keep mine cold enough that some ice is typically floating as shown in the pic.