Depends what you're making, and how you cook it. You have to keep in mind they're usually already blanched so it's easy to overcook. Peas, for instance, you just put frozen in a strainer and run a little hot water through them to melt the ice crystals, then stop. Add them to your hot dish at the end. Making creamed spinach? Defrost your frozen chopped baby spinach just enough to squeeze out the excess liquid, and stir into your hot cream sauce. By the time it's hot again, it's cooked enough.