Agree with hang-the-nine.
I live near the DC area and pretty much receive 139 mbps/12 mbps. I run the test every 3 - 4 weeks or so. No real pattern or plan.
However, the test always goes to a server in DC. Also have a sense that Comcast knows that a speed test is being run. When I change target sites the results are usually still very good.
Right now I am being pestered to upgrade my modem for "better service". I think they just want me to have a modem that will become a hotspot for them. Have not gotten a straight answer as to whether or not I can turn off that function.
Also, as I understand the just received bill, my service plan has been changed and the price raised. I thought I was set for a year or so.....