I don't think anyone knows for sure just how long a zombie can continue to walk around. It's never been discussed in past zombie movies either.
Winter is coming on, I'm wondering how that's going to affect them too.....I think they're going to have a hard time shuffling thru 3 or 4 feet of snow, depending on where they're located.
The book "World War Z" dealt with this. Roughly, the zombies freeze in place.
Good book, BTW. An easy summertime read, something to peruse over an afternoon with your feet propped up and a cold drink sitting next to you.