Who told you to hibernate your snake over winter? I've never heard of such a thing. My thought right now is that you may have gotten some bad information. Snakes might brumate a bit over the winter months, but you don't hibernate them like you do a turtle or tortoise.
Some snakes do hibernate. Most colubrids do, as well as some kinds of boids. I'm guessing this is one of those boids...
First of all, do not hibernate your snake unless he is 100% healthy. Dehydration is common in hibernating animals. It is also of extreme importance that he doesn't have food in his stomach; if he doesn't have heat to digest it, it will rot, resulting in his death.
After the last feeding, give him two weeks to digest, to make sure everything is out of his system. Then turn off the lights and heat. No need to move him into a separate enclosure. Temps can go down to 50*, no lower.
The time he spends in hibernation depends on the weather in your area. He will wake up when he's ready (when he feels that barometric pressures have gone up enough); then you can turn back on heat and lights, and start feeding him again.
Also, it's really important to keep him hydrated. Dehydration is common in hibernating animals. Weigh him every few weeks. If he starts to loose weight, it is his water weight. Soak him in room-temperature water for a while, then let him go back to sleep.
From what I've read (which isn't a whole lot, mind you), you aren't supposed to force them to hibernate once they start waking up. But I'm guessing it would be about three months. :?
I was just reading about hibernating reptiles this morning in the August issue of Reptiles Magazine (don't ask why the August issue is out in June, lol). It was a very well-written article that I highly reccommend, if you'd like to do some reading.
Dont brumate a young diamond, your not meant to.. keep them growing till they are adults, they dont HAVE to be brumated, only kept cooler then all other aussie pythons. Brumation this young will effect him in the long run.