Kurt's body has taken a lot of damage over the years. Because of his devotion to the business, he has never really taken time off to properly rest and recuperate. He's also going through a divorce and an addiction to pain killers which, is a result of all the bumps he's received in the ring.
If he can get his life back in order, you can expect him to be back in wrestling (hopefully, with a lighter schedule) within six months to a year. That is of course, if Angle actually takes time to straighten things out.