Out of pure curiosity, I spent an hour at the Northern Vista Point of the Golden Gate Bridge on a sunny Saturday afternoon. The parking lot was completely full and there was a backup onto Highway 101 South across the bridge and as far back as Golden Gate Park - a distance of 4.6 miles.
I monitored 20 parking spaces, about one sixth of the total, for 60 minutes, and found the following results:
1) 8 spaces were occupied by a single car the entire hour
2) 12 spaces changed cars one time during the hour
3) No space changed cars more than once
4) The shortest time parked was 12 minutes
Vehicles stayed significantly longer than I had expected. This could be because the weather was relatively warm, but it could also be the result of a feedback mechanism where people stay longer when they have to wait longer to get their space.
If we assume that the 20 spaces are representative of the parking lot as a whole, and there is no reason to assume otherwise, then it is easy to estimate that approximately 72 spaces turn over per hour. This is consistent with my observation that it took about a minute for a typical waiting car to move forward by one car length. (I assume that all of the cars that enter Vista Point end up parking. In reality, some might give up and exit.)
If we assume that half the cars stopped in the right lane of the bridge are in line for Vista Point, then there are approximately 150 cars waiting in line to park. A car at the southern end of the bridge would have to wait 2 hours to reach Vista Point and find parking. You can see why they might want to stay for longer than 15 minutes!
There are two sets of restrooms at Vista Point. One set was closed and the other had long lines for both men and women. I would estimate that about 8-10 percent of the people at Vista Point were either in the restroom or waiting in the restroom line. It is likely that the long restroom lines result in longer average parking times. It would be worth investigating whether traffic backups would be measurably reduced if the second restrooms were opened.