Warm...
Setting the Interrupt_Control pin.5 = 1 simply enables the Timer0 interrupt.
That is not setting a pin, it is setting bit#5 of the register.
When the count goes from FF(HEX) +1 (255 decimal,it over-runs back to zero).
Correct.


Now for the tricky part.
Running with a 4MHz OSC the chip is actually running at 1MHz.. Or cycles at 1 micro second (ticks).
If the prescaler is set for 1:1 then it will overflow every 256 micro seconds.
If the prescaler is set for 1:2 then it will overflow every 512 micro seconds.
If the prescaler is set for 1:256 then it will overflow every 65536 micro seconds.