I’m not saying that projection doesn’t do some harm to transparencies but I have often left slides (35mm, 645, and 6x7) up on the screen for several – 10 or even 20 minutes at a time - without any noticeable effect. Projectors have a fan to whisk heat away from the film. I wouldn’t worry about it too much.
Agree. A good projector puts very little heat on the slide because of the fan. My Dad and his friends used to leave a slide up for a long time while they were discussing the qualities of it. Those old slides still look great!
He used a Golde projector, and I have a couple of Aeroquips, and neither one seems to heat the slide much.
Moma don't take my Kodachrome away!
Oops, Kodak, of all people, did!