I didn't realize this before I arrived, but old faithful stopped being faithful back in the 1950's. Apparently it is still "almost every 90 minutes" plus or minus 10. It depends upon the length of the previous eruption. Eruptions less than 2 minutes will cause the next one to be 65 minutes away, and eruptions more than 2 minutes will cause the next to occur within 90 minutes (makes sense, takes longer to recover).
Anyhow - all of these geysers are really cool to watch. There is an extensive boardwalk setup that allows you can get up close and personal with some of them. It's neat to watch the water recede and the hole to become empty, then it slowly rises again and begins to peculate violently until finally it shoots up into the air. Some of them don't shoot into the air, they just keep sloshing water up over the sides like an ocean wave pounding a rocky shore.