I'll try this out. At first glance, though, the ISR is always setting TimeToSave to 5000 so I think it needs a check like:
Well, yes, that was the whole idea. As long as the user keeps turning the knob (generating interrupts) the counter will get reset to 5000 (or whatever). But as soon as the user has NOT changed the RPM for the duration of 5000 iterations thru the Main loop the value is saved. When the knob isn't being turned no interrupts are being generated so the Main routine eventually gets the counter down to 0 and saves the value.

/Henrik.