Simplicity at its finest, to be sure. But the only way to know if a blade needs replacing is to experience an uncomfortable shave. Life's too short for uncomfortable shaves. I know I can get 6-7 good 3-pass shaves out of a blade. New blade (DE) in on Monday, toss it after Friday's shave, before it gives me an uncomfortable shave. Saturdays are for my straight razor shaves. Sundays for my SE shaves.
If a blade needs replacing, it will be obvious at the outset of the shave. Then just replace the blade. Very nearly all of that shave will be comfortable. After two to three blades, the blade life should be known. For me, that turned out to be 7 days with the blades that give me the best shaves. I seldom use blades other than the two best. When I do try a different blade, the same methodology works. Should one ever be better than the reigning champs and be readily available, it will become my new everyday blade when I run out of the existing blades. Maybe it is just me, but simplicity is as important as saving money, the two of which are second only to getting great shaves.
