While the PIC is sleeping Timer0 will be off since on-chip clocks are shut down. You could
use Timer1, but you would need an extermal clock during sleep.

If it will be awake during your 5 minute time period, you could use the compare module set
to reset Timer1 on each match. Load CCPR1L and CCPR1H with 62,500, set a prescaler of
1:16, and it will interrupt every 500mS + automatically reset Timer1 to 0.

Then you can just increment a variable in your int handler until your delay period is reached.
This is a lot easier than reloading a timer in an int handler - or fiddling with adjustments to
reload values compensating for code overhead.