Thinking about it I probably use both sides of a DE equally each shave with them. Tell you one thing I do, I pull the blade out of the DE, or SE, and rinse it off and lay it on the edge of a square scuttle. I pick it up the next time I shave with a contraption instead of a straight. Sometimes I have a sense of how many shaves I've gotten out of a blade, other times it is too long ago to recall. When I start the shave if it isn't smooth and comfortable I change the blade for a new one.
I noticed 30-40 years ago that if I left a blade in a DE it would begin to rust and stain the chrome or nickel on the DE. That was the main thing that motivated me to start removing the blade, rinsing it and setting it aside. Might be better for the longevity of the blade, maintaining sharpness, as well.