I purchased the single standard license for video series. It says that this license lasts for a year. If I stop purchasing it after a year, can I still have the full license of my already-uploaded videos?
Yes, the license is valid for a series of videos up to 52 episodes produced during a year. Remember, this is only valid if the videos are part of an actual series. It can’t be any 52 videos produced by the same channel, they need to be connected.
You can read more about it here https://audiojungle.net/licenses/faq#what-constitutes-series-a
The one year limitation applies to publishing. Episodes of a series have to be published within a year. Once a video is published the license remains valid forever.