Lube

I haven't performed a thorough test, but I'd say oil-based lubes take the longest, and water-based the shortest, simply based on how often I find a dry film of lube under the cap...

Good thing is, if a water-based lube starts to thicken inside the bottle, you can always reactivate it by adding a little water and shaking the bottle a bit.

Let it be noted, though, that I've never left a lube alone long enough for the entire contents to congeal. XD