It'll depend on the toughness of your beard, how good your prep is (good prep=softer beard), the make of the razor blade as some last longer than others and how well you clean the blade off after the shave. Cleaning and drying the blade will make it last longer.

And it'll also depend on how often you shave! I use my DE maybe 4-6 times a month, so the blades last ages. My dad uses his everyday and the blades last him weeks so... YMMV I guess!