About 1 in 730,000. Yellowstones last super volcanic eruption was approx. 640,000 years ago. It's average time between eruptions is 730,000 years. Was a Super Volcano every 600,000 to 800,000 years. That's the scientific numbers, I say maybe a little more dangerous because it erupts every 600-800 thousand years, and it has been 640 thousand already. If we start seeing increased activity and earthquakes around yellowstone then keep an eye on it, but if the volcano activity just settles down and it sleeps for another 150,000 years, don't hold your breath yet.