You can make them up to 2 days in advance, but I'd recommend making the caramel or ganache in advance instead and making the meringue cookies the day you plan to serve them. You can prepare the caramel in advance and pipe it into the center of the cookies instead of scooping it in while warm. The caramel is soft enough that it shouldn't be a problem. You can also make the ganache in advance, and then pipe or scoop on top of the cookies after the meringue cookies are done.