We are both from the Uk where 4 months is about right.

Oh and by 'the longer, the better' i never said that. lol
One of the best ways to get an idea of when you should be hibernating your ants is to look outside around Oct/Nov, if you stop seeing any ants about then its most likely they are hibernating and then so should yours. And then in Feb/Mar/Apr if you start seeing ants again its time to wake yours up.
I find that because your ants are in 'controled' condistions then you can give them the same hibernation time at the same time every year (I do 4 months, 1st Nov to 28th Feb) and they soon learn when it is and expected it, so prepare for it and thus you have a healthly colony. Its better then in the wild as sudden weather changes can catch a colony off guard and weaken or even kill the colony.