It depends on the fixer you use. If you use TF-{3,4} or Ilford Multigrade fixer type of rapid fixers, 5 min is probably very safe. Washing out fixer is a diffusion process and good agitation is important.(posted 8278 days ago)I rinse the film, reel, and the tank and fill up the water, agitate a few times, dump water, repeat for a couple of times, and put the films in running water film washer for a few minutes. (Basically, first half of Ilford film washing method.) This way, I can minimize water wasted because all I need is a continual water motion, not the replacement of water, as residual fixer level in the water is already very low even if I keep using the same water.
Some argue prolonged wet time increases the grain. I am not sure if it is a reliable observation/comment. But I think wet time is less important compared to paper.
For technical information on fixing and washing, see Fixer for film and paper.