For this recipe, you do not need to soak the french toast (unless you want to). However, many french toast recipes call for soaking french toast for at least 20 minutes. If you want to prepare this as an overnight french toast casserole recipe, you can soak it in the refrigerator, overnight, covered with plastic wrap to enjoy the next day. When dunking the french toast in the custard, the individual pieces of french toast only need to be coated in the mixture for about 5-10 seconds.