I've often been able to speak for 15 minutes, up to 2 hours, on topics without repeating information. It was usually in a teaching or on-boarding context. The people were engaged, nodding, laughing, taking notes...
I simply knew the subject and its adjacencies inside out and backward.
Ten minutes, for a subject you care about, seems trivial.
But what I mean is that 10 minutes would be perhaps the approximate boundary where I feel comfortable holding the floor engaging in curious but uninformed speculation, and the sense of fraud that I am possibly wasting everyone's time by continuing to speculate begins to encroach.
If I care about a subject enough I think like most people I don't have problem holding forth on it, free of supporting notes, for 2 or 3 hours if circumstances (and the extent of my command of the material, and the patience of my audience) would allow.