one hour. overnight is better.
coat your grill with cooking spray to prevent sticking. heat grill to 500-550 degrees (direct high). put the skirt steak (still folded) on the grill and close the lid. grill it for 1 1/2 -2 minutes. open the grill to check and make sure you don't have any seriously roaring flames going on; if so, carefully move the steak out of the flames to another section of the grill and close the lid. using tongs, flip the folded steak over, close the lid and grill another 1 1/2-2 minutes. unfold the skirt steak and grill on each side another 45-60 seconds per side with the grill 