DaveReidUK - that I don't know. In my role at the time, we parked up a number of aircraft for FR during the winter when not required. From my recollections, no aircraft was left idle for longer than three days and the handlers / line engineers suggested this was the reason.
I imagine there would be something in the Boeing manuals somewhere which makes this the case but I couldn't quote you a reference. It is why I suggested that if a Boeing engineer was lurking on the forum, they might be able to provide some clarity.