Some geysers erupt unpredictably several times a day, others only at intervals of weeks or months. But Old Faithful goes off so regularly that its eruption times are posted by the park service - you can even check them on-line! Eruptions are a little over an hour apart, and as the predicted time approaches more and more people arrive to watch. The benches go halfway around and are usually full, with more people standing behind, a thousand or more.