The general rule of thumb is to let your sled warm up for approx 4-5 minutes. What most people do is go outside and start their sled and let it warm up, then go inside and change into gear and come back out and ride!
I usually let it warm up nice so i can touch each cylinder and if they are all warm just go a little easy for the first mile or so then progressively open it up and you'll be fine.
5 minutes is fine, shoudl have heat in the pipes and cylinders/pistons by then. Just do as 2ooosrx said, start it, then go in and change into gear, then go out and ride, that usually takes atleast 5 minutes.