How long can I soak red kidney beans for?
Stir and watch for bubbles.

Beans tend to start fermenting after a day of soaking. It's not sure they will, but they often do. They will still be edible, if slightly sub-par if they just began, but a day more and you should toss them.
If you see the soaked beans to "fizzle" like carbonated drink, toss them. If they barely release a minimal amount of bubbles or none at all, cook them thoroughly and you're good. Also, if they start minimally fizzling, make sure to wash them thoroughly. Most of "spoilage" is in the water and on the surface.