We waited for about a half hour for Old Faithful to erupt. It goes off about every 90 minutes (plus or minus 10 minutes), and there was definitely a big crowd for a 2:00 pm Wednesday afternoon show. The crazy thing was that while we waited there were other geysers that were erupting not too far away. It is an amazing place!
The entire eruption lasted only a couple minutes, but it was spectacular. (Well, I thought it was. Doug wasn't as impressed, but he was also really sunburned by the wait in the sun.)
At its peak, Old Faithful reaches about 120 feet. You don't want to be close by!