Not sure on the rule regarding how long left to run on the policy, but I know I got a rebate two years ago when I passed my test (Insurance taken out in January, test passed in July - FYI... Axa were the insurers).
I can't remember the exact figures, but I remember that the difference between Prov vs. Full licence had a fairly small impact on my insurance. It's only with growing NCB that my quote is dropping at a noticable rate.
(For what it's worth, whenever I'm made a change to my policy [e.g. change of location, change of car etc.] I've always recieved an updated cost with the relevant increase/decrease passed on... even when only a couple of months to go on the policy)