... so I would say that objects and buildings that don't date much will have a long shelf life ...
Agreed. It all depends what the photo is of.
I have 35mm film images of African wildlife that I took over 20 years ago. I'm scanning them, and they're selling. In fact some (like images of rhinos) seem to increase in value as the animal becomes rarer.
On a slightly less specialist note, this image was taken 38 years ago ...

... and now sells steadily (though not spectacularly).
On the other hand, a photo with a model in that I took a just a few years ago is a dead duck now because the clothes look so dated.