Questtion was asked: Assuming they did fuel it, how long can it stay in that condition?
Your guess is as good as mine.
When the U.S. does a missile test, a lot of things have to go right and scrubbing a launch is expensive, so you usually don't wait once you have clearance to launch.
The last test I was involved in, the range clearance window started at 20:00:00 ZULU. Range safety gave verbal clearance over the radio and launch happened at 20:00:05. There is absolutely no advantage to holding on to a missile that's ready to fire, only bad things can happen while you're waiting.
Maybe HipShot can answer that question, it IS his field, after all....or was before he retired. My observation might be to go back to the early days of the US Space Program, say Mercury or Geminii. How long did they let those birds sit on the pad before scrubbing a mission. I recall that one of them was forced to go to the bathroom in his suit rather than break him out of the capsule to find a facility. That bird flew, too. I guess it was Sheppherd and after he "went" they lit the candle!