Burst is a feature in MikroTik RouterOS that can allow users exceed Max-Limit for a limited period of time.
It is a great feature to improve customer satisfaction when using burst type of application like HTTP or E-mail services. To make speed throttling fair for an ISP router will make sure that burst will be enabled only if average user consumption do not exceed a threshold value.
Router will calculate an moving average value in Burst-time period, and if that value will exceed burst-threshold values then burst will not be alowed anymore, however when user will stop using its service then average will eventually go back under threshold value and user will be able to use burst again.
Becouse actual time for how long burst may take is bit complicated, you can use below calculator to estimate usage and see it on simulated graph