When to remove batteries?

[fiirmoth]

I have several collectible DS and 3DS devices along with a few super rare PS3 controllers(Like the Uncharted one). I have recently started worrying about battery leakage. The DS and PS3 controllers never get used. At about what age does everyone start removing batteries from these devices to ensure they dont leak and cause a problem? I try to pull everything out and charge/let run about every six months but does anyone have other methods/ideas on this?