The maximum punishment for that is complicated, but as far I can see it is either max 3 months or it can be sent to the Crown Court where it is max 2 years.
Possibly extradition to the US over Wikileaks stuff, but no pubic application for that yet. But if he could be extradited from the UK to the US over Wikileaks, why would the US do the convoluted "extradite him to Sweden then on to the US from there" scheme that he claims was in progress? (ducks)
There are obviously open cases as he has been arrested.