Always fix for the least amount of time necessary to do the job. The longer the film is in the fixer, the more it soaks in, deeper into the emulsion (and paper, if making prints) it gets and the harder it is to remove. The more fixer remains in or on the film, the greater chances of degradation as time passes. One of the biggest problems for film preservation is degradation because enough of the chemicals weren't removed.

You're using rapid fixer. In my experience, five minutes ought to be enough time with rapid fixer. This assumes that you are using fresh fixer, not recycled, that you use it at the correct dilution and with the right temperature and agitation. If you do everything "normally" and according to the manufacturer's instructions, you've got all this covered.

However, if the instructions say to fix for ten minutes and you otherwise follow the instructions, ten minutes it is!

There are ways to determine the exact amount of time needed to correctly fix film, using a clip of undeveloped film and a stopwatch but, for now, let's just do things by the book.

Other than that, just put the film back on the reels, put it into the tank and pour in the fixer, just as if you were doing it the regular way, for the first time. Make sure you rinse the film the way you usually do and use rinse agent if that's part of your process. Hang the film up to dry as usual and proceed normally from there.

I suppose you could cut the film apart and redo just the parts that need it. You are probably going to cut the film into short strips anyway. Right? However, it might be wise to refix everything. It could be correct to assume that, if some of the film, didn't get fixed properly that there might be other parts that weren't fixed either. A prudent person might just refix everything just to be safe. How much extra work is it, really?

Whether you do it today or tomorrow, still, check your reels to be they are clean, dry and undamaged before you process any more film. An ounce of prevention is worth a pound of cure. Right?