@JeremyECrawford I cast Invisibility on myself. I cast Invisibility on myself again, before the duration of the first spell lapsed. Did I become visible?
— Eric Green (@quadhund) January 12, 2018
A concentration spell ends the instant you start casting another concentration spell, and a spell takes effect when its casting ends. If you cast invisibility in succession, you're visible during the 1-action casting time—effectively for a split second or so. #DnD https://t.co/thzQYzDT0U
— Jeremy Crawford (@JeremyECrawford) January 12, 2018
Wasn't it 3 seconds for an action, 2 seconds for a bonus action and 1 second for reaction with movement in between per turn? You'd be visible for longer than a split second there.
— Ilya Donskikh (@IlyaDonskikh1) January 12, 2018
A round in D&D lasts for about 6 seconds. None of the things that take place during a round—actions, movement, and the like—have official amounts of time assigned to them. #DnD https://t.co/tufUKubsvF
— Jeremy Crawford (@JeremyECrawford) January 12, 2018
1 action = 1 split second, word of god confirms Don't forget the last part of the sentence: "or so."
— Jeremy Crawford (@JeremyECrawford) January 12, 2018