My knots usually hold up pretty good right up to the point where I get a bite.
This is actually a thought provoking question. I've always thought it reasonable to retie when the line was strained or worn, but I have never given much thought to loss of knot strength over time. I have left knots in place for weeks with no noticeable loss of strength. I use uni knots for just about everything, so maybe that knot doesn't put as much stress on the line.
I would be interested to hear if anybody has actually had a problem with this. To be a valid observation, it would have to be a case where old knots failed repeatedly and then the problem was solved by tying fresh knots. Anybody?