Assuming you start with Timer0 clear, in counter mode, with a 1:1 prescaler, you will need 256 clock transitions on T0CKI before it will generate the over-flow interrupt.

Also, you don't want to set INTCON = %10100000 when using PBP's ON INTERRUPT. Just clear the Timer0 int flag before exiting the interrupt routine with INTCON.2 = 0. PBP will handle GIE for you.