Do not leave these mushrooms out for more than 2 hours at any point in time. If your mushrooms haven't been cooked and need to be kept on hold a while longer before baking, place them on a baking sheet and top with plastic wrap and keep chilled for up to 2 days before baking. After the mushrooms have been cooked, do not let them sit out for more than 2 hours without placing them in the fridge or reheating them.