Personally I think that the helicopter scene was just to show how long it took for the walkers to start migrating(note that Glenn actually commented on the herd migrating-just to labour the point). The passing of time in TWD is another subject worthy of debate. To be honest I'm not really sure how long its been since the za took hold, but I don't think its as long, as it seems, as the season doesn't seem to have changed although I live in the UK and I dont know what the seasons are like in that part of the US.
It started mid to late Sept according to the Rise Of The Governor novel. I'm guessing it's mid to late November possibly even early December in the show.
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