Usually three; depends on the blade (Atra seems to last the longest). As has been pointed out, DE blades are so inexpensive it's really not worth the effort to try and squeeze more than 3 or so days out of a blade to save another $.20-$.50 - certainly not with all the damage a dull blade can wring. When I was a young lad (before carts were invented...) I'd try to squeeze a week out of a blade - and I'll note that there'd be quite a bit of surface rust by Day 7.