The answer to this really depends on two things: how fresh your ingredients are and how frequently you use ranch seasoning in your foods and recipes.   The fresher the herbs and ingredients used, the longer your ranch seasoning will last. In fact, this seasoning mix could last as long as 1 year when stored in an airtight container in a humid-free environment. I suggest that you start off by making this seasoning as per the quantities listed in my recipe, making a note of when you made it, and then seeing how long it lasts. You may find that you need to make a double batch next time or require less depending on your own needs and uses for it.