How often you change your blade depends on how you treat it. If you hit the side of your ruler or run over a pin, you will create a dead spot in the blade. It will need to be replaced.
Cleaning and oiling your blade on a regular basis will give you a lot longer life. After every project you finish, pull your rotary cutter apart and wipe the build up of lint away. Put a few dabs of machine oil on a soft cloth and wipe blade. Reassemble your cutter.