Dan(9698) is an ex-missile guy. He might be able to answer this one.
The missiles I was involved in were anti-aircraft. The first was the Nike Ajax. It was liquid fueled, and could stay fueled for months. The oxidizer was red fuming nitric acid, which is a liquid that did not require either pressure or refrigeration.
The other was Nike Hercules. It was solid fueled, and so could be ready to fire indefinitely.
Their missiles are liquid fueled, and probably use liquid oxygen as one of the fuels (oxidizer). It boils off if it isn't either under high pressure or refrigerated.
Missiles normally do not have either high pressure tanks, or refrigerated tanks. Both are too heavy.
The space shuttle uses insulation to reduce the loss of liquid oxygen, but is normally fueled just prior to launch.
They calculate how much will be boiled off, and so know how much is left to complete the mission.
They could let the liquid oxygen boil off and top it off just prior to launch, so it cannot be determined how long the missile could sit there before it is launched.
Guessing, it would be a matter of weeks at the most.
Yeah, that is why I pinged HipShot. He might be out of the net for summer vacation. I dunno. But he was a MIRV guy, too.