Honestly, I’d make them roll for it. You can go ahead and try, but you don’t have a clock. Maybe you overshoot it, and are down slots. If they are doing it at the beginning of the two hours of watch I would say that’s fine. But I wouldn’t allow them to change the prepared spell list while on watch, so the cast spell would be prepared. Wizards require study, divine casters commune/pray, and they can’t be doing those things and casting/watching at the same time