It's 6 months actually, but a good question. Not 50 metres from my front door are two UK RHD cars dumped on a footway and communal hard standing with Bulgarian plates from the same province. One has been there for 9 months. The one on the footway one month. There has been a local purge on untaxed UK reg cars in the last week, but these two get away with it. The police will only act if the cars are spotted actually being driven.