Brawler works similar to Berserker Rage - after the initial activation, the game starts a timer that checks after 65 frames (45 frames for Berserker Rage) whether the conditions for another activation are met. Afterwards, the game checks for activation 60 frames on each subsequent try.
Minimum duration: 65 frames (1.0833 sec)
Subsequent duration: 125->185->245->305->365 frames etc. (+1 second each time)
If the buff drops, the next time it´s activated, you get 65 on its initial phase again.