As a host, I try to make sure all invitees know they are free to visit
for as short or as long a time as they like. If I'm inviting
people by email I will phrase it something like "the event runs
from 2 pm to 9 pm but feel free to stop by for any part of it
if you can't make the whole thing".

As a guest, I would try to make sure the host is comfortable
with a short-ish visit if that is what I want to do. But I
normally don't double-schedule dinner.

Beyond that, there are no rules.

Steve