Actually you can probably get a nice polish on the silver in a couple of hours. I never tumble my metal work for 24 hours. A couple of hours tops unless I am doing head pins or ear wires in dead soft. Then its closer to four or five hours simply to harden the metal so it keeps its shape.
I use about a half a pound of shot and fill the water to cover the shot and whatever pieces are in the tumbler by about an inch or two. You don't really need to fill it full, it makes the motor in the tumbler wear out a lot faster and it doesn't make any difference as far as the polishing time or effects go.
If you silver shot is rusting at all its not stainless and I would replace it. My stainless steel shot has been left half covered for days (not on purpose and not by me lol) and there isn't a speck of rust on it after two years.
I am new to making beads but not to metal smithing in case anyone was wondering lol!
Sonja
www.silverriverjewelry.com