If you followed our advice and completed every other mission before Reaper IFF, you’ll be able to (read: have to) finish Legion’s loyalty mission, after which the main story will advance. This timer is variable - the more missions you have unfinished, the more misisons you’ll need to (read: be able to) complete before the story compells you towards its conclusion. Remember that after you finish Reaper IFF, you’re on a timer before the next phase of the main questline begins.