Sprint’s general recommendation for fixing time is a range of 1-3 minutes. Fixing beyond 3 minutes is not recommended. High-speed liquid fixers, such as Sprint RECORD Speed Fixer, will fully fix any standard emulsion within 3 minutes.
The general rule of thumb is to fix for twice the minimum clearing time. Try this simple test. Put a piece of undeveloped film in a small beaker of fixer working solution (1:9). Stir, and note the time needed to clear the film of emulsion. Double the resulting time to establish your base fixing time, i.e. 60 seconds to clear = 2 minutes fixing time.
When the film clearing time has doubled, it is time to replace the fixer.