This question was prompted by this answer to another question by Jamiec.
As the magnetic poles drift (usually pretty unpredictably), runway numbers become inaccurate. As noted in the linked answer, this has resulted at the very least in the renaming of the 15/33 runways at Madrid to 14/32.
How often does this happen, and at what point is it no longer reasonable for pilots to make the corrections necessary to make a smooth landing, requiring a renaming of the runways?