I've had it last two weeks before... but sometimes it's a little sour by the end. Goat milk freezes wonderfully, for what it's worth. Since it's naturally homogenized, the fat doesn't clump up.
I generally try to only thaw about a week's worth at a time, so I don't have to hide the sour taste with chocolate syrup.
It really depends on how it's handled. From the cleanliness of the milking set-up to how quickly/thoroughly it's cooled, to what temp it's kept at. Your best bet would be to ask the farmer how long it tends to stay sweet in their experience.