I am making a cake with a buttercream crumb coat, and a MMF. Anyone know if the cake should be kept refrigerated due to the butter in the BC? If not, how long will it be okay to leave out of fridge?

I freeze all my cakes that will be covered with MMF for at least an hour. The fondant is so much easier to smooth when the icing is not too soft.

Qali, you might want to chill/freeze your cake after you've crumbcoated it in buttercream and before you put the fondant on it, like CAKELADIE1 suggests, but there's no real reason to keep buttercream refrigerated all the time. The sugar acts as a preservative and will keep the butter from going bad for weeks, even.