As is the advice for any popular museum or attraction, the earlier the better. I made a point to head out early so I could leisurely make my way over to the Met, right before it opened. I got to the steps fifteen minutes before they officially opened and was surprised (I shouldn’t have been, really) to see a small line up already forming by the doors. As crowded as it seemed outside, it disperses once you get into the sizeable museum, so you have maybe half an hour to an hour until throngs of people really get in your way depending on where you are.
Go early. You’ll thank yourself for it.