I usually load several guns at the beginning of the season. The load stays in the gun until it's shot during the season, or until my first range session with that gun after the season. Sometimes that's a week or two after the season end, sometimes a few months.
As long as it was loaded on a clean dry barrel (and I mean fully clean - no fouling shot - no caps popped) I would have no qualms leaving the load in for a year. In fact, my brother-in-law did that last year with his Knight.