Depends on capbadge I believe. Most seem to drop out at 5-8 years, last time I was told about it, with technical trades being at the top end (why not sign off once you have a load of supervisory experience and civie quals in some cases?)If I remember correctly, the 5 year point had the highest numbers.
Although the 12 year Short Engagement contracts have only been running out over the last year or so. Not sure how many are being converted into Full Engagement, so that could change things.