Based on this study, anywhere between about 50 and 100 minutes.
33 to 77 minutes in this study.
There seem to be quite a few studies out there, but these two are the most recent, and used trained cyclists.
Edit to add: what is noteworthy is that the variation between subjects in these studies is equal to the variation within a given subject in others. That means that a subject who stopped the earliest and the one that lasted the longest could actually have the exact same true TTE - one just had a bad day while the other had a good day!