This is truly on how often you use it. I personally change mine every two weeks but i do not have a lot of hair to shave on my legs. When I was younger with a lot of hair on legs I would say once a week.
Usually about once a week. But it really depends on how often you shave. To keep your razor alive longer shave with baby oil or the baby oil gel. Plus that will keep your skin much more hydrated and silky smooth.