I try to get the most mileage from my rotary blades but sometimes have an OOPS! moment that forces a change. For instance, last week shortly after putting in a new blade I ran off the mat and onto the metal top of my cutting table. That did it for that blade.
I attended a demonstration by a Clover rep about sewing notions. One of the things he said about needles stuck with me. "With the cost of machines these days, why risk damaging it with an old needle? Consider the cost of servicing or replacing the machine against the cost of a new needle." I know he represents the company selling needles but I think that makes sense. I'm changing needles a lot more often now, especially on my sit down longarm. I've been buying needles in bulk to cut costs.