I always thought it was 7 years for every human year but sons dog has just turned a year old and everyone is saying "ooh he's an adolescent now". But that would make him quite a few years older.
Thers is no exact comparison. For example some giant breeds only live till 7 or 8, so a two year old dog has lived a quarter of its life. Some smaller breeds live till their late teens, so would be at a much younger comparitive age.