By happy circumstance, I was able to draw for her and her friends for an hour (normally, I have a two-hour minimum for parties) for two reasons---it was a small group (which meant everybody got a full color caricature!) and it was actually on the way to my second gig, a wedding reception in the evening. Yay serendipity!
The reason most caricaturists have a two-hour minimum is because then we can get a decent number of guests done at the event, and it helps us to keep from driving between too many short events in a day. Most gigs are far enough away that it just makes sense. I have an unwritten policy that my drawing time must be at least marginally longer then my driving time to get there, one way. The further away the gig, the more hours I require. The craziest trip I had between gigs once was an afternoon party in central NJ, and an evening one in Lancaster County PA. There was a lot of driving that day!