Looking at last year's start times (which were posted), it went by category - and perhaps by when you signed up within that category.
DH and I registered - both in Masters - since the men's Cat 5 - by far the largest group - goes last, and we'd have a big delay for sure if he went Cat 5. So...from the looks of last year, he should start ~1 hour before me, which means he'll be finishing up just about as I start. So we'll have some puttering around to do, but not too bad.
Now...to get myself mentally prepared for this!