A rotary blade rarely fails outright. It goes gradually, and the first thing you notice is usually not the blade at all — it is that your blocks have stopped finishing at the right size.
The three signs
It skips threads. You run the cutter down the ruler, lift the fabric, and a few threads are still holding at intervals. That is the classic tell. The blade has flat spots.
You press harder. If you have started leaning on the cutter to get through four layers, you are compensating for an edge that is no longer there. Pressing harder also pushes the fabric sideways under the ruler, which is where accuracy goes.
The fabric shifts. A sharp blade slices; a dull one drags. Dragging pulls the top layer out from under the ruler by a thread or two on every pass. Over a dozen cuts that adds up to a block that will not square up, and most quilters blame their piecing rather than their blade.
How often, roughly
For most quilters cutting quilting cotton, a 45mm blade lasts six to eight projects. Cut through batting, felt, laminates or paper templates and it will be far less — paper in particular dulls a blade faster than anything else in the room.
Nothing about this is precise, which is the point: change the blade when it behaves badly, not on a schedule.
Buy them in packs
The most common reason people quilt with a dull blade is that they only have one, and replacing it means a trip. A ten-pack costs less per blade and removes the decision entirely.
Our rotary blades are priced per blade on every listing, so a five-pack and a ten-pack can be compared without arithmetic.
One more thing
A blade with a nick in it — from hitting a pin, or the ruler edge — will leave an uncut thread in exactly the same place on every pass. That is a nick, not dullness, and no amount of sharpening fixes it. If you see a repeating gap at a regular interval, the blade is finished.