I occasionally need to deal directly with the General Managers of the divisions in our office. Usually these are quick things that warrant a small disruption of their day; requested project understanding, updates, and general things that an e-mail or a phone call can't explain. In some cases, even sending an e-mail or a phone call to ask about a quick meet gets bogged down by other happenings.
Naturally, I go to their office/space for a couple minutes to solve the issue or show them how it works. Occasionally, they are caught up in someone else's impromptu meeting with no indication as to when they'll be finished. Note, their office door is open, or the person is obviously not having a prolonged stay so it's not a matter of "maybe they have a planned meeting".
Obviously I don't want to stand there for 15 or 20 minutes waiting on them and removing time I could be working. I'd also like to not come back every 20 minutes until they're free, but I definitely need to get in contact with them and physically explain to them whatever is going on.
I usually wait a couple minutes before announcing that "I'll come back in a bit", but this seems to not work as well; these are busy people. Any messages left on their phones or e-mails will go ignored for either a longer period of time (usually days) or completely passed over with so many other messages.
Is there any rule of thumb on how long to wait for someone to finish an impromptu meeting?