With film you can reasonably do the twice the clearing time method.
Test with a scrap or piece of leader before processing, or else, check the film after a minute in the fix, then perhaps every 30 seconds afterword. Note how long the test strip or rebate takes to be completely clear, then extend the time by how long that took. You'll probably be at 3 to 4 minutes total in fresh fix, somewhat longer as it's used.
The real method would be to use a residual silver test which is mentioned in the link.
1. Fixing time varies with the particular film. Fast films take longer to clear than slow one. So checking with HP5+ is good. Also the fixing time increases for a bath with its use. Within reason over-fixing does no harm. This means that fixing times can be doubled or oven tripled without harm.
2. Usually fixer is discarded when the clearing time doubles. The bath has become saturated with silver and its continued use risks archival permanence.
3. Fixing rate is also determined by whether ammonium thiosulfate is used or sodium thiosulfate. Other differences in fixer formulation have only a slight effect. The blanket recommendation is 2 - 4 minutes for rapid fixer (ammonium thiosulfate) and 5 - 10 min for conventional fixer (sodium thiosulfate). Stay within these times and you should have no problem.
Rapid fix time is usually 2 minutes, regular fix time is 5-10 minutes, the longer time is for T grain a it not only takes a fresh bath but clears t a slower rate.
Rapid fix time is usually 2 minutes, regular fix time is 5-10 minutes, the longer time is for T grain a it not only takes a fresh bath but clears t a slower rate.
No, as fixer is used the fixing times slowly increase even for rapid fixer using non Tgrain films. That is the reason for the upper time for both types of developer. If you continue to use only 2 minutes with ordinary films the results will not be archival as it exhausts.