According to the documentation on bursts:

"the average data rate is (0+0+0+0+0+0+0+512)/8=64kbps, which is under burst-threshold. After the second second, average data rate is (0+0+0+0+0+0+512+512)/8=128kbps."

Then burst time = (burst limit * poll time)/burst threshold and I should be able to calculate the burst time for actual rates based on averages, but this doesn't hold true.

I want someone to burst for 5 minutes (300 sec) at 5120k and my threshold is 512k then I should be able to get the burst time that I need for this by: (5120k * 300s) / 512k = 3000s

The kilobits even cancel out properly. So I should be able to use these settings and get 5 minutes at 5120k and then 512k from there on out. If the router only polls every 1/16 of the burst time then the burst should be disabled at 375 sec (6.25 minutes).

The problem is that this doesn't work! I tested it. Go ahead and try it yourselves and then get back to me.