Can I make popovers ahead of time? If so, how? If not, how long will they last after I make them?

Make sure to make a slit in the top to let the steam out, so the popovers don't get soggy. Reheat gently.

Agree, popovers should be eaten as soon as they come out of the oven. They're just not the same if they sit.

Popovers are best straight from the oven. But if you don't mind a slightly deflated popover then you can make them up to 4 hours ahead of time. I'd rewarm them in a 250 F oven for a few minutes before serving.