According to Mike Francesa, the Jets are giving Favre until the NFL Combine to make his decision. This year's event takes place from February 18 through February 24, so that gives Favre little more than a month and a half to make his decision.
There are also rumors that Favre does not require major muscle surgery, and he just has a minor muscle tear. It's only on a message board right now, so it might be unsubstantiated, but the poster says he read it on 1010 AM. Here's the link.