The burst available is determined by how long the traffic stays under it's configured average. The port builds up a "burst bonus" while it is under the average speed. A burst event consumes accrued burst bonus until it has been exhausted at which point the port is now constrained to its configured limitation. Once consumption drops under the configured average, the burst bonus starts to accrue again and the port can then pull from the bonus to burst again.
Think of it like a gas tank that only fills up when the port remains under the configured average for a period of time. The gas is only consumed during a burst event. Once the gas is gone, the port can't burst.
As for how long it takes to refill the burst bonus, I can't say definitively, but it will be a function of the average port traffic utilization and time. The less utilized it is, the quicker the burst bonus will replenish, and the longer it remains less utilized, the more of a chance there is that the burst bonus gets replenished.
From the docs, regarding burst size:
The maximum number of bytes to allow in a burst. If this parameter is set, a port might gain a burst bonus when it does not use all its allocated bandwidth. Whenever the port needs more bandwidth than the average bandwidth specifies, the port can temporarily transmit data at a higher speed if a burst bonus is available. This parameter tops the number of bytes that might accumulate in the burst bonus and be transferred at a higher speed.
