I looked it up in the contract and it's covered in section 23.S.11.

11. A reserve pilot who flies on an X-day due to an IA, GS, or reroute will be given nine hours free of duty upon his release at the completion of his rotation.
a. b.
His X-day(s) will begin immediately following such nine-hour period and will continue until he has received a period of 24 hours free of duty for each interrupted and remaining X-day in his scheduled X-day block.
If the remaining days in the bid period are insufficient to contain the X-day(s), the pilot will be granted an additional day(s) off under Section 23 S. 16.

In summary: A reserve pilot that is flown into X days gets his payback day as block in +9.5 hours.