I thought I'd apologize in advance for the absolute beating your tires, brakes, and mental state are likely to suffer at our event at Mosport next week.
I've just finished the first draft of the 'schedule' (I use that term lightly).
A typical day at NHMS includes about 90 minutes of seat time for each run group. That's 4 sessions of about 22-23 minutes each, and as you know, some of this time used for the checker/cool-down laps and such.
The schedule for Mosport includes approximately 330 minutes (5.5 hours) for day 1, or approximately 4x that of a typical day at NHMS.
Day 2, the TT day, is likely to be in the 4+ hour range.
8 hours is a conservative number. It's possible to hit 10, but hey, let's just call it 8 for now. Figure a typical lap time of 1:40 on the 2.459 mile circuit. In 8 hours, that's enough to turn 708 miles (288 laps)!
A few years ago I turned 525 miles at COM's 2-day event at Mosport (200++ laps). Gordon drove a bit more and beat me by a handful of laps. Looking back, we both were probably standing around the paddock too long and not in the cars enough! I suppose it does take time to refuel, swap tires around, and other essential pit-stop activities.

Here's to rejoining the 500 club at Mosport (and reaching for the 700 club!).
Bring tires, brakes, oil, gas money, and sleep!