I can only speak from an aircrew perspective which is 12 months but I do believe that this is common across the board. Interestingly, the rules changed not too long ago whereby, the ability to give 6 months notice from the age of 50 has been removed.
The line now is that those with 30+ years of service MAY be able to leave after 6 months depending on service needs. A colleague at WAD who PVR'd on his 54th birthday was refused permission to leave early (in the last 18 months), so Manning are clearly happy to invoke the small print clause when they feel the need to do so.