We bought our passes a few months before we used them back in 2012 when people were thinking prices would go up before our May trip. Our first day in the parks we arrived mid afternoon, so there was no one at the ticket booth so we did the whole exchange for our passes there and even though there was no line - to do up 5 passes still took about 10 minutes.
It's something you do have to do eventually - so I'd suggest when ticket booth lines are short.
You can buy them now and take till December 31st to activate them if you wish - and yes, the clock starts ticking when you either trade them in at the booth or go through the gate with the eTicket.
Have fun!!