I fell 3 weeks ago and hit my right side of my head on an office chair. i have a bump now, how long until its gone?

It should be gone by: Most bumps like this, are gone and have resolved over 2-3 weeks. Sometimes a bump may be pre-existing, but the injury can bring it to your attention. So watch it for 1-2 more weeks....if it does not diminish in size(protuberance), you should go see your doctor for examination to make sure that it is not a tumor of some type which may have to be removed.