My blades last forever. I only change them when I think they are going to die of old age. I think the angle that you hold the blade does make a difference and I try to cut at different spot on my mat. Once I made a Buggy Barn pattern, and had to cut 18 layers. Changed the blade that time, but it cut just fine. I use Olfa and an Olfa mat. Also try not to hit the edge of your ruler. I think that dulls the blades.
Sue