I’m a little unclear on one fact here, although I’m not sure it matters.
It sounded like you volunteered to photograph an event and that that event was supposed to last five hours. If so, you’re agreement was not time-based, but was a promise to do a reasonable job of photographing the event. In the same way your “employer” would have been out of line in suggesting you’d failed to live up to your agreement by using the wrong shutter speed (unless you’d done something unreasonable in choosing one), it was also up to you to determine how best to take pictures of the event. You did this, even if you were not shooting for all five hours.
Even if your agreement was to shoot for five hours, it sounds like you stopped shooting when the event was over. So long as that’s the case, I can’t imagine that your agreement could reasonably be interpreted to say that you needed to spend 15 minutes shooting caterers packing up, just so that you hit your five hours.
Think of it this way, if the event had just run 15 minutes short, would you still have owed them?