How far ahead can this be made?

The butter should keep well for at least a week if tightly wrapped & refrigerated. I wouldn't make the sweet potatoes more than a 1, maybe 2, days ahead. Again both tightly wrapped and refrigerated. Reheating the potatoes in an oven safe dish covered with foil should be pretty easy. You may want to add a little bit of liquid to the potatoes for reheating so that some steam is created.