Was reading about recovery from total knee replacement and it can be a bitch.
If you are back in the gym after only a month I'd say your doing very well.
Many go back to the gym right after. It really is going to depend on a variety of factors:
1) were you fit or a "slob" prior to the surgery
2) Did you do PT PRIOR to the surgery
3) Diligence in doing home PT in between the PT visits
4) Continued work on the legs after you're cleared
I know quite a few masters competitors in crossfit with TKR that recovered fast.