You leave it at home in the jar unless your ride is REALLY long, REALLY hot, or REALLY wet (i.e. pouring rain).

In those cases, you might want to apply a light, thin coating on any parts which might encounter friction (i.e. where your legs meet your torso). If you are female, there's an additional spot to apply it as well.

Remember, you can always reapply it along the way, so you don't need to load up with half the jar or anything.