I've never considered how long a rasp is meant to last. I have a Folgers can full of assorted styles and sizes that have come my way over the years. My favorite is a semi-round roughing rasp that has produces maybe 15-20 bows. Due to screwing up a bicep several years I seldom use a drawknife any more. Mostly a hatchet and a nasty toothed farrier's rasp. Farm auctions, estate sales etc. are good places to pick up draw knives and rasps cheaply, especially if you happen to live near an Amish Community.