I doubt if they would cancel your reservation until all are boarded or 10 min before departure, whichever occurs 1st. And in overbooked situations, they usually will get volunteers in advance -- although these are only used if the plane acutally does fill up.

Best bet, if multiple connection flights available, get a 2 hr. overlap to compensate for delays (or at least 90 percent of delays arrive within 2 hrs. late).