I have numbness in my left hand that I got on a 1200km ride last summer. I blame it on a road with expansion joints every 30 feet for miles and miles. It has finally subsided after 7 months, but it's still there to a small degree. I wouldn't say it ever was really weakness. I find that it almost always goes away after a month, sometimes as short as a week. But it can be permanent, so the best idea is to avoid it if possible. Seat back and longer stem takes weight off of the hands. Of course, that might not be your most powerful position, so tradeoffs are necessary.