This is a question I should be able to answer but, for the life of me, I cannot remember the exact rules. I thought it was 3 years after your first day of soldiering, so the day you joined Phase 1 training.
I will check... unless some Arrser cleverer than me comes along!
SOLDIERS' CAREER STRUCTURE
All new recruits sign on for a full 22 year career under the Open Engagement. From 1 Jan 08, new recruits will be enlisted on the Versatile Engagement (VEng)
They must normally serve for a minimum of 4 years or 4 years and 3 months reckonable service dependent on age at enlistment) including basic military training and subsequent specialist training.
Employment will initially be with their own cap badge but, as experience, qualifications and promotion are accumulated, increased opportunities arise that allow for posting away (i.e. to training posts or other specialist jobs); this is normally followed by a return to the unit. It is possible for soldiers to either remain with their unit for the whole of their military career, or be trickle posted from one unit to another, dependent on the cap badge.
Once Soldiers have completed the full term of their career, the opportunity to extend their service beyond 22 years is open to selected individuals who still have valuable experience and skills. There are two main forms of continued service available.