The Old Faithful doesn't erupt on a strict time table but on an average day the Old Faithful Geyser erupts 17 times per day.
It's a very pretty and awesome sight to see when you watch Old Faithful erupt in person and if you haven't seen Old Faithful in person I highly recommend you do so.
About 90 percent of the time though the Old Faithful can be predicted to erupt within 10 minutes so it does become predictable as to when the Old Faithful tends to erupt.
The time between eruptions of Old Faithful varies but is between every 60 to 110 minutes.
Very awesome sight to see in person and you never really want to leave and want to see it again and again.
Or at least that has been my experience.
I have got to go back and see Old Faithful again soon.