To answer your first question, for me, I notice a blade needs to be replaced when it starts grabbing and pulling my whiskers. I can get 4-5 days out of most blades.
For your second question, as cheap as blades are, I wouldn't worry about flipping them. I don't think its worth the time and trouble to do so, and I don't think it would matter as extending the life of the blade.
I have moved recently to DE shaving. Having normally only shaved every 2 days I find that I get two good shaves per blade. The third shave starts to drag and hurt a bit. This tells me that I will have to use two blades per week and I feel I have quite light growth.
Like Dustini says the blades are cheap so don't worry about cost. Listen to your face.