A lot of slides were used from the 50s to the 90s for instructional purposes in places of work and remained on a screen for several minutes at a time while the instructor explained the purose of waht was on the screen and were used repeatedly for many years.

These were probably not Kodachromes but were E6 processed slides and in my experience they seemed to withstand the experience very well.

So are Kodachromes that much more susceptible to projector light and to the extent that as little as a minute will ruin them? Surely Kodachrome holiday slides and the like were shown time and time again and when Uncle Harry was explaining what the scene was and how much he and Auntie Hilda enjoyed exploring the antiquities etc it is difficult to believe he was switching off the light after say 30 secs or moving to the next slide.

I recall seeing the same slides( probably E6 admittedly) time after time in the late 50s when families got together and some slides were on the screen for several minutes at a time.

Those slides didn't appear useless or even show any signs of deterioration when shown for the second and third times as they often were.