For Desktops with small spindle, I know TJ says about 3 hrs is the limit before having to do warm up again.
If I've gone on off on a tangent for 2 hours and know my next cut will 2 hours with a .25"engraving at 17K or whatever, I'll give it a three minute shot at 15K and 2-3 at the actual RPM. Overly cautious in a 55-65F shop, but can't afford a new spindle so I baby it.
Just tossed it in for Desktoppers searching.
scott