I don't think storage conditions have much to do with it, though heat does speed up chemical reactions, so warmer conditions may speed it up a bit. Honestly I'm surprised so many of these cells *don't* leak after so many years. It's remarkable that one could go 20+ years without leaking the caustic stuff inside. Personally I would not leave a battery more than 5-6 years old in a system. 10 years is pushing your luck, longer than that is just tempting fate. If the machine will be stored for a long period, just be safe and pull the battery. It's not like there are a ton of settings that need to be restored.
I bought a stereo receiver off of ebay which still had the original batteries in the remote and in the receiver for memory backup. The unit was likely put into service in 1986-87. Batteries didn't leak and the remote still worked enough to turn the unit on/off!