Short answer, about 3-4 minutes each side. However, grilling time depends on how hot the grill is and you have to adjust accordingly. On a gas grill, I usually grill about 3-4 minutes each side. On a charcoal grill, I grill the mushrooms for 5 minutes each side as I tend to put them on the sides where there's less heat. Also, on a cast iron grill pan, I usually grill about 4 minutes each side after pan has heated.