I have found that it depends on the blade and your Mat... I have an old Olfa mat (10 plus years) that a couple almost instantly got a nick in from this mat. But, the same thing happened after a fashion with the more expensive blades so I figure at a dollar a blade I can toss some sooner than others. When they were 5 bucks a piece that ticked me off more...
I stop at the local HF when I pass and pick up a few packs everytime. Yesterday I picked up 5 packs, and yes I left some for other quilters.....