From the GeoCities geyser web page, I quote

Old Faithful erupts every 35-120 minutes for 1.5-5 minutes to a height of 90-184 feet. The rangers say that 90% of their predictions are within +/- 10 minutes. The time to the next eruption is predicted using the duration of the current eruption. The longer the eruption lasts, the longer the interval until the next eruption. For instance, a 2 minute eruption results in an interval of about 50 minutes whereas a 4.5 minute eruption results in an interval of about 85 minutes. It is not possible to predict more than one eruption in advance.

Surf the web to find potentially more useful data