Well, instructions about capacity of the chemicals are important. Whatever you do take a note and observe results of your actions. With tray chemicals usage counts because they have quite specific capacity. Especially counting is important with the fixer as it doesn't change color with the use and effect of "semifixing" LOL, come out way to late. Like few months later, in the form of reddish, blueish or browniesh spots on the print. Used up fixer is a silent killer. With developer it is simple, works or not. If not, time to change. Stop bath indicator is risky to. It is changing color, but to late, after it's capacity to neutralise alkali is completely gone thus contributing to death of the fixer. And your prints. Change it each time you change the developer. To extend life of fixer I set up fourth tray with just plain water between stop and fixer, but it could be second stop to. There are also indicators for checking the usefulness of fixer. Bad fixing nullified the whole effort of making print.
Ha, not only how much oxygen is in the bottle, but how much was absorbed while in tray. Developer won't survive long, one day. Fixer much more. Stop should be OK for long.
Look at your developer, if it is somewhat brown, is gone. Phenidon based developer will survive little longer than metol (PQ versus MQ).
But with fixer better count usage. It doesn't change the color, it doesn't change the smell. especially the one with hardener.