I change my blades out every Monday. I usually get 6-7 shaves out of them. I've found if I try to go longer the chance of cutting myself goes up dramatically.
I haven't counted; I just go by feel. When I think I'm getting a poor shave, I change the blade out. I'd recommend using more than just water. I don't know if shaving cream will reduce wear and tear on the razor, but it will certainly do so for your scalp.
I have really thin hair...so I get about 30 days out of each razor.