I have a "black eye" resulting from my concussion and basal skull fracture previously described. how long before my "black eye" goes away?

About a month: Depends on volume of bruising and absorption. Blacken, then green, then yellow then gone like most bruises. Some local heating pad use may quicken the resorption.

Up to 4-6 weeks: Depending on how much blood leaked into your eye socket, it may take your body up to 4-6 weeks to clear that blood out of there. If only a little bit of blood is present, it may clear up within a week or two.