It starts to loose potency, as the bud is exposed to any sort of light, especially sunlight. It also is a result from lack of care when stored in baggies with air, you need to keep your bud stored in an airtight container, or at least a bag that has nearly all air sucked out of it to retain the best qualities of potency.
But on the side note, you can keep marijuana potent for several month's by just keeping it in an airtight container with a normal temperature and darkness for a long time.. I don't know how many month's it could stay potent before degrading THC. Another alternative is to store your bud's in a freezer, as long as its sealed properly in a Mason jar, or something with almost no air circulation, your bud's can stay potent for long periods of time, but I don't really suggest it, because I would've smoked it by now.