Sometimes it's the mood, but usually its when I'm down to my last 500 cards. If I have energy to do it, I redesign, or change things around. But if I'm swamped with work etc, then it's a re-order that happens.
I order business cards in batches of 3,000 and they usually last me 3 months or so - so about every 3 or 4 months I'm changing something around.