I had the same issue with my old 2 focus's. It would work fine, but then when you go back to the car and start it up with air con on again, it would mist all the windows up really heavily and take forever to clear.
Best solution I found was to switch the air con off about 5 minutes before the end of your journey to let the pipes etc come back up to normal air temperature before switching off, this would stop it misting up again next time. Could be something to do with condensation forming in the icy cold air ducts when the car is sitting which then is blown out over the windows when the car is re-started.