I can tell when a bit is getting dull from the quality of the cut. You’ll get a little bit of hairiness from a sharp tool, but a dull one will give you a really ugly cut that you’ll spend forever cleaning up. I typically change out bits anywhere between 20-40 hrs of cutting depending on how hard I’ve been on the tool.
Examples! 
'Cause if you’re like me, you need visuals.
This is what I get from a fresh tool:
These parts were cut with a tool at the end of it’s life:
