More personal, it's about how you feel the bur.
Once it's burned and doesn't cut as well, you should either throw it out or use it to refine your prep (like I do haha!)
The reason it isn't great to use a burned bur (not sure if that's how they're called but whatever) is that they will cause tons of friction/heat (when used to cut; for refining they are great because they give the prep a polished look), which negatively affects the pulp