It greatly depends on the jam, how familiar people are with the terrain, the number of people, the skill levels of the participants and a few other things.
Heres my jam break down based apon the 4 jams I've been to
20% walking around scouting/familiarizing/being shown the favorites of the locals
70% drilling favorite spots/weak techniques
10%filiming cool 'runs'
Now if you kept jamming with the same core group of people who were familiar with the area and had similar skill levels you could concievably run for as long as your endurance could take you. But I think for the most part you spend the jam hammering out moves to string together.
All of the films you see online, aren't just 5 minutes of hard core PK, but rather entire afternoons spent getting film worthy runs, or at the very least are mere snippets of a couple hours of footage cropped down to show the tricks the authors want to display for comments and criticism.