Just checked my router stats for my SamKnows box (FibreMax) and its used around 260GB so far this month (202.3GB Down / 57.9GB Up) - so averages out to about 780-800GB per month usage without any skipped tests. 

As others have mentioned, its runs at various times of the day/I have my main PC connected to it and doesn't run the test if its seeing usage from other devices - Don't notice any difference/impact while its running.