28 weeks/days
They don't feed, not really. Its more like they spread their zombie condition. Primal reactions in species. Mammals have teeth, they bite. Zombies are only usually effective in numbers, its not the feeding that is important. It's the bite itself. Once bitten, it doesn't matter after that. If the victim doesn't get away on time the bite just leads to eating, as well they keep biting until beyond a doubt death has happened. Or the victim stops struggling. I doubt it's ever about the food. Just the spread of the contagion. Feeding is just natural continuation of the first bite.
or in the second case the corpse would try and re establishthe spiritual energy and soul that they lossed upon there death by consuming other living beings.