There's no exact rule of thumb here but anywhere from 20 minutes to 24 hours is a safe bet for marinating chicken. If you're using a marinade that has a particularly acidic profile (think marinades heavy on lemon juice, vinegars, etc.) then you will want to marinate only 20-60 minutes and not overnight. With milder marinades such as honey mustard, pesto, BBQ and wine-based marinades, you can let them sit overnight in the fridge.