The stop bath (usually dilute acetic acid or even water) neutralizes the developer and stops the development process almost as soon as the film becomes in contact with the stop bath. There is no point to continuing the stop bath any longer than this, but it will do little harm when development occurs in perfect darkness. When developing with a safelight, you want to minimize the amount of exposure to the safelight until the fixing has started, so you want to make the stop bath time as short as possible and get the film or print into the fixer right away. (The same neutralizing effect also happens without a stop bath, if you were to go straight to the fixer, but then a great amount of the fixer would become "exhausted" and requires you to use fresh fixer much more often.) Therefore use only as much time in the stop bath as is needed to be sure the film has come in contact with it, usually 15 - 30 seconds. (Often that's the time it takes just to pour the stop bath in and then out of a developing tank.)
If you leave a film or print in the fixer too long, I recall reading that it can oxidize: it will turn brown and lose contrast. (It might be interesting to run a few experiments with blank film strips or bits of photographic paper. :-) It is important not to under-fix, because that can leave the film slightly sensitive to light, causing it to darken and lose contrast over time. There is a lot of latitude, though; my experience is that most fixing occurs within the first few minutes (or even seconds, with fresh fixer at a high temperature) and you can keep film or prints in the fixer for ten times as long without harm. The difference between 5 and 7 minutes is of no consequence. (I haven't observed any problems in my negatives even after 40 years and I recall not being too fussy about fixing times.)
A good technique to get the most out of your fixer, and to be surest of the results, is to maintain two bottles of fixer: one old and one new. Use the old fixer first, right after the stop bath, then after a minute or two, switch the film over to the new one. Eventually (after many uses) after the old fixer starts looking cloudy or brownish, discard it, cycle the "new" into the role of the old, and make a new batch of fixer. This is easiest to carry out for fixing prints, because you simply add one more tray to the pipeline (developer--stop bath--old fixer--new fixer--wash).
In short, you need to pay close attention to procedure (timing, temperature, and technique) during development, get into the fixing stage as soon as possible, and then you can relax. Just remember that you have a film or print sitting in the fixer: take it out before your next break and don't forget about it altogether!