According to How Stuff Works, razor blades need to be changed when the blade starts pulling the hair or if there is rust on the blade. Typically, a razor should be replaced when it no longer glides over the skin or if it does not properly remove hair.
The amount of time a razor blade lasts depends on a number of factors, including how often it is used and how it is cleaned. For example, a razor that is not properly cleaned can rust or become dull from shaving cream and hair build-up. After using a razor, clean it in warm water, let it air dry and store it away from water.