...Take note of how many seconds it took for the film to go completely clear, then double that number: that is your optimal fixing time, for that brand/dilution of fixer. The "clearing time" increases as you use the fix for each roll of film: the fixer becomes saturated with silver and so the amount of time it takes to remove silver from a film gets longer and longer. Why do I suggest you learn how long the ideal fixing time is, using this method? Because if you simply drop your HP5 into fresh Rapid Fix (of any brand) and leave it there for 2-3 minutes, you are allowing the fix to penetrate the emulsion far more than you need to. If you fix for only the minimum ideal time, then the washing stage can be completed in a shorter time and fix is more easily washed out of the film. In fact, freshly made Ilford Rapid Fix clears HP5 in as little as 10-15 seconds in my experience, and so the film is fully fixed after 30 seconds under normal conditions. There's quite a difference between 30 seconds in the fixer, and three minutes. (or more!) Bottom line: there's no need to fix longer than the ideal minimum. Find out what the ideal is for your fixer and film combination.
Some would say this is "a fine detail" that isn't required learning for someone first starting out with their own processing, and perhaps that's true. But it doesn't hurt to learn good habits from the very start