Have a look to picture attached

At 75% of MaxVO2 power (near FTP), you will be depleted in a bit more than an hour
At 60% of MaxVO2power (not far from IM pace for elite), depletion will take between 3 and 4 hours

Of course this will also be depending on :
the way you train (impacting if muscles will use more fat or CH at the same % of MVO2Power)
the way you trained your muscles to store (riding low frequently for exemple)
and the way you were supplied before the ride.

If applying correctly LCHF (low carb high fat), CH reserve will last much longer as muscles will use higher % of fat at same % of power.

So the answer is more between 1 and 6 hours depending on race pace (Sprint to IM race pace) and other parameters.

It is not too difficult to evaluate your consumption for each race pace.

I was talking in the context of long rides which I think was clear from my post, but perhaps not. I should have been more specific.
Anyway, I reckon for longer rides I would typically be doing around 65% VO2max which appears to extrapolate to about the sort of time range I mentioned based on your graphic.

Yes, riding at around 65% of Max VO2 power will lead to depletion around 2h30

It's not a time thing, it's a kcal thing. From what I've read, the typical number is about 2000 kcal, with 1500-1600 being muscle glycogen and the rest being in the liver. But not all of that is available for cycling, muscles can only use 'local' glycogen so your legs don't have access to that full 1600 kcal. The exact numbers will vary based on size (but then again so will expenditure) and training status and diet.

So even when fully topped-up, 2.5 hours sounds pretty optimistic to me for anything but a pretty easy endurance ride. I for sure wouldn't do a 2.5-3 hr road race without taking on additional calories.