The answer is: "that depends." It depends on what you're marinating and what you're marinating it in. While you want maximum flavour from you marinade, you need to be aware that leaving some meats in a marinade will break the meat down too much and leave it slightly mushy. A stronger, more acidic marinade will require less immersion time than a less acidic marinade. For example a buttermilk or yogurt marinade will take longer to work than a vinegar or soy sauce marinade. Dense, hearty meats such as flank steak and roasts require the most marinating time (8-12 hours on average), chicken and poultry needs less time (3-6 on average) and fish, especially 'flaky' fish such as trout or tilapia requires very little time comparatively (1 hour). With the exception of light fish, almost no cuts of meat will suffer by marinating overnight in the fridge. However, you will need to experiment a bit with different recipes to find your optimal marinating times.