Traditionally it used to be the cutting of the cake if there was no formal send off (birdseed, petals, bubbles, etc). Those are still true for older generations (40s and older) who are used to those being the end of the wedding with nothing after, but they have since become photo ops instead with much more party following.
As a guest, do you stay to the very end when staff starts cleaning up or do you use one of the above events as a sign the party is over? Or something else entirely?