Marijuana seeds should be keep in a cool, dark place such as a basement or in your refrigerator. Heat and moisture 'signal' to the seeds that it's time to sprout, so as long as you keep the seeds in a dry, dark environment they can remain viable for years. I have heard cases of seeds sprouting after being in storage for 5-6 years. However, you will notice that older seeds take longer to germinate then fresh seeds and a few of them may not geminate at all. As time goes on, fewer and fewer of the seeds will successfully germinate.