Things may be a bit different at the moment but I left in the back half of 07. I found my desk officer to be very flexible with my exit date. I started talking to him with just over a year to go and moved my exit date to the right by 12 months in the end. They only actually have to do formal paperwork at 6 months to go so they do have some leaway. Just be aware how long it takes to fit in all the resettlement courses though. It takes longer than you think to process the paperwork and get on the ones you want and you can't start until you have given notice. Took a bit of doing cramming it all into 6 mths.
Feel free to PM me if you want more detail. All the best.