You could just do it by trial and error, let it run for a while and see if and how much it drifts. Otherwise you need to dig into the generated .lst file and count the actual instructions. I'm guessing somewhere around 16 cycles.

Or, why not use a second timer (TMR0 etc) to actually time it. Stop TMR1, start TMR0, do the reload, stop TMR0, start TMR1. TMR0 will now give you your answer.

If you tick the timer with the main oscillator it matter if you run at 8kHz or 8MHz, a cycle is a cycle.

/Henrik.