The timing is normal (for the conference season and Queen's speech). The duration was longer than normal, and of course we're in the middle of a crisis with a ticking clock. A two-week prorogation might have worked fine. But the intent

> what exactly is the PM supposed to (or not supposed) to do?

Normally this kind of conflict doesn't arise, because the government has a large majority, and if they cease to have a majority then the PM is replaced by a no-confidence vote.

Normally the PM is supposed to Be A Good Chap and Do The Decent Thing, which are ill-defined concepts from British honour culture, but have basically gone out the window with Johnson's behaviour.