What is the proper notation for a frame rate that is less than 1FPS?
Frame per minute is totally fine. Frame per hour, frame per day. But I am not sure you can keep going using an acronym indefinitely.

But when talking about timelapse, you normally do not talk about FP(add letter here) but intervals. So the term is actually different than FPS.

The Interval is 1 second, 30 s or whatever.

When reproducing a "video" with this frame rate, I would not think of it as actual video, but more like a slideshow, so again the concept is a bit different.

20 slides per minute, or a slide every 3 seconds.

This is a nice question actually.