Nowadays, unfortunately, there are no rules except, I would say 1 hours prior, specially on domestic flights because some revenue standby can pop up. On international flights it is different first in general standby is not allowed and some markets, specially in South America are know for their tendency of no shows. There is also another factor, you priority rank because some employees with high priority can also pop up at the last minute. I would say, if you have a low priority 10 to 15 for domestic and 5 for international.