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Anyways, to the OP, I'd say that if you want to hit them with their armor off, like in the middle of the night or in early morning, simply tell them that they don't have time to put their armor on. It's as easy as that. If your players ask why, tell them that it'd take too long. You don't need some rule making up numbers for you to show your players- you know your group, make something up that suits all of you.
Although there is in fact a rule about putting on armour. PHB, page 212:
"Putting on a suit of armour always takes at least 5 minutes, which means that it’s an activity you can undertake only outside combat"