First: Timescale, one turn equals 2 years on the timeline which can be found when clicking your factions banner, but a character only ages 1 year per turn (or maybe even per 2 turns).

Second: When being sallied you can just quit the battle immediatly and nothing will happen, if it was the last turn before the siege would be won by you, you simply win the siege. This makes fighting sallies rather irrelevant.