Depends on how much water you're using.

I use a very scientific method - I fill a metal pie tin with hot water, and add pinch or two (literally - a thumb/forefinger pinch). Stir it a bit to dissolve, and add the rusty bits. Then let it sit and occasionally check it for progress. I've done soaks for a couple of hours for lightly rusted stuff, and for a couple of days to take care of really rusty stuff. I tend to err on the weak side, just because.....

I had some cable clips with light surface rust that came out gleaming after about a 2 hour soak. Conversely, I had a chain that was dry and bright orange - after a 2 day soak it looked and worked like new.