Use it regularly if only to keep the needle from gumming up - typically I'm squeegying the stuff
through a solder stencil so I just unscrew the needle anyway as I need a sizable blob to do this,
so I often find the needle is fairly dried out, which usually clears with a lot of force applied and a
bit of patience.
Fridges are often suggested, which may be OK for small amounts, but you should have a healthy
respect for the rule of never putting anything containing solvents in a fridge or freezer. Firstly
they are poisonous and should not be near food, and secondly many solvents evaporate even
when very cold, causing an explosive atmosphere in a box with an ignition source (the light