while it's impossible to say exactly how long a multivitamin is effective understanding how different vitamins are absorbed and stored in the body can give you some idea. generally water soluble vitamins (b vitamins and vitamin c) aren't stored in the body. instead they are absorbed based on need with the remaining excesses being excreted in the urine. this is why it's important to get these vitamins on a regular basis. on the other hand fat soluble vitamins (a d e and k) need fat present to be absorbed and are stored in the body for later use (fat tissue and liver). so basically some of the vitamins are excreted daily while others are stored. for more on the difference between fat and water soluble vitamins see this webmd article <link>
how long can i take a multi vitamin and it remain effective?