I was lucky and only spent a total of about 24 months holding but many of my era spent between 3 and 5 years in the hold. My advice to you is make the most of your time holding by getting on as many courses/Expeditions etc as possible. You are in a unique position where you are being paid large sums of cash with no real job to do. The courses you can get under your belt now can stand you in good stead in the future, when your primary job commitments don't allow you the time to do them.
I know you are probably desperate to get onto the front-line and operational but you likely have little choice about the hold so make the most of it. Many out us out in the Air Force have been where you are now so understand where your'e coming from.
It is worth it in the end.