Marinades can be made with a variety of ingredients so how long it can stay in your fridge will depend on what is in your marinade. If your marinade is mostly made out of something acidic like vinegar or lemon juice and has some oil or fat, it can stay in your fridge for a good two weeks to a few months. Without the fat, it can last for at least a year. Make sure that your jar of pre-made marinade is uncontaminated by bacteria. For it to last longer in your fridge you have to keep it in an airtight jar and always use clean spoons to scoop out marinade.