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:

Code:
mission	cease_hostilities ; (NOT SUITABLE FOR VARIANTS)
{
    POPE_MISSION
    image_path_issued	pope_mission_issued
    image_path_expired	pope_mission_expired
    image_path_success	pope_approval
    image_path_failed	pope_mission_failed
    duration 5
    score_modifier 0.8
    turn_start		3
    paybacks
and change the 'duration' from 5 to 10. This should make the mission last a longer number of turns.