Stop bath and fixer can be reused, for film I mix 600ml at 1/4 and fix ten films per the 600 without trouble but for Tmax I would halve the amount, A quck test for fixer is to use the film leader and when the leader takes double the time to clear compared to fresh fixer dump it, Stop bath use untill it changes colour, with Developer it is far simpler to use it as one shot and dump it after every film.