To some extent it depends on what you put in the butter to make it a compound. For instance, truffles won't handle freezing very well.
But your average, run of the mill compund, like maitre d'hotel, lasts pretty much indefinitely. It's a sort of a universal, "good-housekeeping" recommendation to use anything in a home freezer within six months; but I imagine you could go easily go a year without losing much quality in a stable freezer.
BDL.