The USDA recommends 6 hours up to 24 hours. It is safe to keep the chicken in the marinade for up to 2 days but the meat could possibly be altered in texture or flavor. Personally I feel 2 hours is the minimum time if you are in a hurry, longer is better. I have never marinated raw, unfrozen chicken past 12 hours since I feel the meat starts to get a bit mushy. And the marinade should be discarded when the chicken is removed.