Depends what rules you are abiding by.
A re-animated corpse would be unlikely to function for long without respiration/nutrition.
Even if there is some nutrition available, how efficient a decomposing body will be at using it is questionable
I would say 2 weeks would be a push!
I reckon a dead body would be pretty useless after less than a week. Being exposed to dirt, grime and the elements, decomposition is going to occur quickly. They may not end up as small puddles of mush, but I reckon enough tissue will slough off and/or be eaten to make it unable to move, let alone be a threat.
That's before we get on to whether it is cold [zombies will freeze] or very hot [zombies will dessicate]. No, traditional walking-dead zombies hold no fear for me.
The still-alive rage-zombies found in 28 Days Later or I Am Legend, now those are zombies to tool up for!


Or more seriously I'd arm myself with the biggest sharp object I could find then head over to Windsor Castle (or the nearest location with armed guards) and find a rifle/automatic weapon or two. Shouldn't be too difficult, although it may involve having to get into the castle proper.