How long does a landlord have to return a deposit after I vacate a property.?

I vacated my rented house in Northern Ireland at the start of June. As yet the landlord has not returned my deposit. We have spoken to him a few times and although he is deducting some for oil (as we couldn't afford to put oil in the tank before we moved out), we are expecting part of our deposit back. It has now been exactly a month to the date and nothing has turned up. How long does a landlord legally have to return the money?