It might only happen when it becomes crystal clear that Johnson is prepared to drive us off the no-deal Brexit cliff.
The problem is that it's already too late. If Labour called a motion of no-confidence on the very first day back from summer recess, the timeline means the resulting election would still be after Brexit day. Boris's first day as PM was Corbyn's one and only opportunity to try to force a pre-Brexit election, and he chose not to take it.