If you take the mission it will not expire but the CB that grants reduced vassalization cost will after 10 years. I'm not sure what causes the mission pool the refresh but each time you will risk loosing (or rather not getting again) the mission.
Okay, thanks. Don't CBs get extended once you're at war, though? i.e. If I DoW Teutonic Order 9 years after taking the mission, will I have more than 1 year to still be able to qualify for the reduce vassalization cost from the CB?