For the fixer, the standard rule is to find how long fresh fixer takes to 'clear' the film. This might be, say, 2 minutes. You can carry on using it until this clearing time doubles (in this case, to 4 minutes).(posted 9235 days ago)Film developer is more difficult. I used to use T-Max once only. Then I read the instructions, and noticed that Kodak suggested you could reuse it twice (for a total of three times). I have tried this, and it seems to work. I increase development time by 20% for the first reuse, and another 20% for the second.
I haven't done proper testing on re-use, such as using a densitometer on the blacks. For critical work, I don't use second-hand developer.