I shave 5-6 times a week (not on work days off), have a normal beard and change my blade (blue Gillette Twin) evey 30-60 days whether I need to or not. I have gone 3 months with just one blade (but it was getting dull by then).
Since I shave in the shower with soap - at the end of the shower - my beard is really soft (I also use hair conditoner on it if I am in a hurry). When I shaved over the sink with commercial lather (without softening it first), I changed my blades every 3-5 days.