It depends

What is the time recommended for fixing your prints / how fresh is your fixer / how strong is your fixer / FB or RC - these all affect the answer?

The old rule of thumb was 1/2 the total time - but that always had a good margin of safety.

If you are using fresh strong fix like Ilford Hypam at 1+4, then 10 to 15 seconds is enough when the total fixing time is 1 minute.

Martin

Martin