Goodness, I only change blades when absolutely necessary. I never kept track, but I believe it is probably every 5--10 quilts. I use an Olfa mat and either Fiskar or Olfa blades. I recently bought some titanium blades and really like them. I wonder if you are hitting the ruler with the blade and that is dulling them. Another thing, I don't cut too many layers at a time. Usually 2--4.
Sue