For me, it’s easy. When the blade no longer cuts the way I want it too, it gets changed. It has nothing to do with time, or distance, if it isn’t cutting the way it was when it was new, I change it. Like was mentioned above, I have even changed a brand new blade because it did not cut the way it should.
At the very least, I replace the blade before starting another project, I replace blades after each session, I replace the blade when the cut starts getting ragged, I replace the blade when it won’t track properly. This is why blades are sold in bulk…. Packages of 144 or a gross.
I think once you start changing blades more often, your scrolling will become more enjoyable.