Currently, it is a yearly event. I have been advocating for the two ‘endless’ battlegrounds (Korrak’s Revenge and Classic Ashran) to be the ‘off-week’ brawls. I know I’d stay subbed for much longer if I didn’t have to wait an entire year and MAUs would surely benefit - I know people who have been in Endless Ashran for 8+ hours a day.
https://us.forums.blizzard.com/en/wow/t/the-big-battleground-feedback-thread/818765