About 760,000 years ago the Long Valley Caldera in eastern California produced a supereruption. A study in PLOS ONE read the history of its last moments from chemical patterns recorded in the rims of crystals in the magma.
Just before an eruption, pressure in the chamber drops at an accelerating rate as dissolved gas escapes from the magma. The crystals show that no more than a year passed between the end of the chamber's recharge and the eruption, in line with an earlier study of Mount St. Helens. If that holds for other supervolcanoes, the warning time may be about a year. It is a single study, so the result cannot yet be generalised.
via We May Have Just A Year To Prepare For A Supervolcanic Eruption | IFLScience