It depends on the fleet. On the domestic side it's usually for 1 or 2 days. Those sitting MD-11 Reserve, however, can expect to be called out for 14 days. Most domestic fleets have an 8 day and 7 day block while the international guys have 15 day blocks. I've been on multiple seats and have never been called out for more than 3 days at a time on the 727 or 747.