When I have trouble cutting down the whiskers by the corners of my mouth. That's a tell tale sign for me I need to change the blade.
Simple
Is it Monday?
Yes-new blade
No-old blade
Well throw wrench in mix, kid you have NASTY THICK beard it is harder on blades then some guy who is Blonde/Blue, and fine soft beard.
Time is teacher to when blade is not working like you like, then you change.