ime using purigen 24/7 over several years, to me color is still the best indicator of when to re-charge. Do it before it turns very dark brown, whether that takes 3 weeks or 4 weeks, or 6 weeks or more, in your particular tank.
As I mentioned earlier, I re-charge mine about every 3 weeks, and have re-charged many bags up to as many as 12 to 15 times before I get the sense that is has outlived its useful, effective life, then replacing with new.
So a 250 ml jar can & does do me for 4-6 months, using the full jar of beads spread among 4 bags, 2 of which are in use in both of my AC filters, while the other 2 are being re-charged, and then allowed to sit in conditioned water awaiting their next usage.
The evidence of the efficacy of that routine, is that my tank water remains crystal clear all the time.