Most rain jackets marketed as waterproof should be completely waterproof for at least a few months, if not years, depending on the frequency of use. If used in extreme weather regularly, the waterproofing on the jacket can wear off within 3 to 4 months. The type of waterproof materials used on the jacket materials can influence the longevity of the waterproofing, and no jacket will be waterproof forever. You can re-waterproof the materials if you notice your jacket losing its waterproofing.