Wait 72 hours until you start worry about dead yeast.
Also, by "fermentation activity" do you mean airlock bubbles? Because that's not fermentation activity. Take a gravity reading after 72 hours. If the gravity has dropped from the original gravity then you're fine. If not, then you can worry about repitching.
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Bottled: English Barleywine (brewed 9/26/09 -- bottled 5/5/10)
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