[Alan Anderson]

The PIC executes ten million instructions per second. If your interrupt service routines don't use them all up, the main program will be able to execute. Counting instructions is tedious and error-prone, but it'll give you the answer you're looking for.

A less tedious way would be to set an output pin high when you enter the interrupt service routine, then set it low as you leave. An oscilloscope on that pin will tell you how long and how often the routine runs, and will give you a reasonable indication of what fraction of the time you're spending servicing interrupts.