Mine took five weeks.
I post this because I found precious little information when I sought it for myself. Lots of "time will tell" and "it depends on the severity of your ding, your overall health, physical therapy and RICE treatments" etc that never got to something that read "four to six weeks" for example.
So here is one puffin's tale:
-Got slammed in the thigh by my SUP board (whilst getting worked by a closed out wave).
-Was able to continue SUPing (following five minutes of swearing) but after the drive home found walking difficult.
-Major swelling around the impact zone (of my thigh) later that day.
-After several days of semi-limping, developed bruising everywhere BUT where the board hit.
-Tried windsurfing two weeks later...I could not jump the board, and found myself limping again the day after.
-About the time my bruises cleared (five weeks) tried another session. I was 90% good, which is fine by me.
-I still have a bump at the impact site, and it's still sore, but it continues to slowly improve.
So if you've googled the question (HLDADTBTTH?) I hope this one example is of use to you. YMMV.
(The thigh, 2.5 weeks after getting slammed.)