Time is required for the chainmail (1000) and archers (1066) events.
I haven't ever tried to increse the length of mission but this may work. Open the 'eras\data\descr_missions.txt' file and find the following:
and change the 'duration' from 5 to 10. This should make the mission last a longer number of turns.
mission cease_hostilities ; (NOT SUITABLE FOR VARIANTS)