I had the exact same idea, and also wanted to try out the new Classes. Been searching for the same information, and the best thing really is to take the previous years, as the basis for what comes next. Guessing that it will be April? Might be wrong.

The gap between seasons is always different, you'll just have to keep an eye out on the Announcements.

My guess is a new season will start with the release of Land of the Morning Light in Korea but nothing is confirmed.

Some items are expiring on March 8, which may indicate that this could be when they are planning on the release of the Land of the Morning Light or at the very least the next season. This is pure speculation but looking at item expiration dates can help give hints on when they plan on releasing a new event. Until then, you can enjoy BDO as it is and believe me, you will still have a blast. :)