I also suspect this isn't a real world value because if you find yourself in an upside down car, chances are it's because you were in an accident. This means the vehicle likely acquired some damage and therefore would likely have a compromised support structure for the glass.
Rollover crashes are generally pretty safe assuming the roof doesn't collapse and squish you to death, and you are wearing your seatbelt. The energy dissipates without you suddenly coming to a stop like if you hit a tree. Most of the damage will be from broken windows/debris causing cuts/scratches.
If you are not wearing a seatbelt, you run the risk of being ejected from the vehicle, and the higher risk of being 'partially ejected' from the vehicle, which is often worse.
I personally observed only 1 vehicle where the roof caved in on rollover (I believe it was a mid 2000s mustang). If the driver had been a few inches taller, I imagine he would've been hurt pretty badly.
The only other time I saw a roof collapse in wasn't from a roll over, but striking a tree roof first at a high rate of speed.
The only rollover crash I ever observed that the driver couldn't walk away from was a rather fortunate partial ejection. The vehicle only rolled onto its side, pinning the drivers arm between the road and the door as it slid for a few dozen yards.
The durability of the laminated safety glass should not be understated. A fire department's extraction team's approach is likely to be "cut through the glass with a circular saw".
Granted, all of this is anecdotal, with a very small sample size. Either way, wear your seat belt.
