Will a Customer Journey beacon fire on a page if the page is closed too quickly?

The beacon should be sent as the browser is closed. The easiest way to validate that in the Blue Triangle portal is to capture/view your sessionID and you should see the last beacon show up shortly after the browser is closed. The beacon might not fire if a page is closed too quickly, as the tag might not have had enough time to be called or complete processing data for the beacon. If the browser has an unclean shutdown, the beacon will likely not be returned. 

Also, the beacon should fire after 5 minutes of non-activity. According to Google, that essentially means up to 5 minutes with no PC interaction at all, because the slightest touch can trigger the timer to rest depending on the browser used