With a single razor and my current blades, I change the blade once a week. Easy to remember and full proof. However, what do you do ifyou have multiple razors in your rotation? How do you keep track of the number of shaves for each razor/blade pair?
I generally use a blade for a week, but since I switch razors frequently I just guesstimate. If I start to use a razor and the blade isn't cutting well, I change it.