I never have the timer on as its too restricting especially during seiges.

Personally, I would just have quit the battle and considered it a morale victory. then marched back and renewed the seige against a much weaker garrison.

Ideally, what I would like is a system where a battle lasts all day and night and is then suspended for another campaign turn. That would enable armies to be withdrawn or reinforced if no result was acheived, but thats obviously not going to happen in this game.