Worst case, with good weather, you can stop service for a few days: day 1 mandatory rest; day 2 fly crews to where they need to be to start service; day 3 mandatory rest; day 4 return to service. Then start rebooking passengers and picking up the pieces. Carriers with long haul international may need longer, and maybe you need more rest days to ensure everyone is ready for their normal shift, but that's a reasonable napkin estimate.

Otoh, Delta seemed to have recovered after about a week, and canceled about 1,000 out of about 4,000 flights for several days. It's way better to fly 75% of the daily flights than not. There's less wiggle room in a summer schedule for weather, but there's still some wiggle room.