It has historically blown in a big way every 600,000 years or so, and the last apocalyptic-sized blast -- the equivalent of 10,000 Mount St. Helenses -- came about 600,000 years ago. Yeah, I know. Certain doom for Denver's 300 days of sunlight and perhaps a nuclear-style winter or even a Mile High Ghost City.
But maybe not.
While the seismologists at the University of Utah have tracked two significant earthquake swarms in the park in the past couple years, there is no sign that Yellowstone will blow like it did 630,000 years back, or like it did 2 million years ago (the biggest supervolcano eruption ever). The caldera is simply not hot enough for that kind of action right now.
More likely, any near-future Yellowstone eruption would be on the scale of one Mount St. Helens, certainly fucking up DIA as badly as the Eyjafjallajökull has fucked up Heathrow -- but no nuclear winter or 24-inch hot-ash blizzard in LoDo.
For more, check out the Yellowstone Volcano Observatory's FAQs.