I think I'm using a 4 or 5 blade razor right now, but I can make each cartridge last at least a month, easily. I always meticulously dry the blade when I'm done with it so rust doesn't pit the blade. Also, after a month or so when a blade cartridge is starting to feel like it's dragging on my skin, I can get another week out of it by using my forearm as a leather strap. I place the blades onto my skin near the underside of my wrist and push it up toward my elbow with moderate pressure -- with he blades pointing backward of course. This seems to knock any jagged edges down.
-- Let me tell you a story about why I believe anecdotal evidence. --