burst of course is something different than op asks for, no?
Your burst treshold simply defines, that once speed decreases under the treshold value, queue is allowed for burst speed download/upload, for a given period of time.
But - op (original poster) wanted to let user to download at the speed he pays for, but after 5 minutes, to lower it to some lower speed, and let it that way till the end of the hardware ...
I would probably try to experiment in the following manner - marking connection (prerouting, tcp, port 80 = http downloads), marking packet, putting it to e.g. http-downloads address-list, setting desired period. Then, for the user, I would create child simple queue, limiting http protocol to desired speed, if it is NOT on the address-list .... Could that work?