i got curious about this, so i did a little research.
here's a timeline of known yellowstone VEI 7 and 8 eruptions, from the earliest until now, in years ago. if not otherwise noted, the years ago depicts a VEI 8 event.
12,000,000 (VEI 7)
6,400,000 (VEI 7)
Supervolcano - Wikipedia, the free encyclopedia
now, this isn't a perfect gauge, as these didn't all occur in the same spot, and yellowstone is a fairly big area. however, these were the massive eruptions in the area. the apocalyptic ones seem to strike somewhere around every two million years, if i'm looking at the data correctly. there was a VEI 8 640,000 years ago.
i'm not a geologist; maybe someone with any background could tell me what they think this means, if anything.