If the stop bath has an indicator in it (solution is yellow when mixed, orange in the concentrate bottle), you can use it until it changes color to a blue/purple, indicating it has lost enough acidity not to be reliable in quickly stopping development. My experience has been that a liter of stop bath mixed according to package directions (I use Kodak Indicator Stop Bath, because that's what I have and it was cheapish) is good for dozens of rolls before it starts to break down.
Fixer doesn't change color when exhausted (that's the stop bath, if it's the indicator type); you need to track the capacity. Most fixers, mixed to paper strength, are good for something like a dozen to two dozen 8x10 prints, depending on your methods (two-bath is good for a dozen, then promote the second bath to first and pour a fresh second; that gets you two dozen per liter, net). On film, the usual rule is to test the clearing time when the fixer is new, fix for at least twice the clearing time, recheck clearing time periodically, and discard it when the time to clear the same type film has doubled. This will typically be about two dozen rolls of film per liter of film strength fixer.
For developers, capacity depends on the developer. I use Dektol 1+3, from old enough mix that it was brown when I first added it to water; I've had the stock solution for most of a year now (it was a five gallon size) -- and I've never seen detectable exhaustion with up to twenty-plus 8x10 prints in a session over as much as six hours (though I make no attempt to save the developer for another session). Time in the tray affects developers, as well as prints processed, however; best to use factorial development (watch for emergence of a mid-tone area, then develop for some multiple of the time that takes -- 5x is usually about right), and replace or replenish the developer when development time has about doubled from freshly mixed.
