How Often Does Total 'Data Received' Reset in Activity Monitor?
I would imagine a whole lot quicker if the total is in KB than GB

Empirically, every time I DL another 0.01 GB once my download figure is showing in GB - that's roughly every 10s, at 10MB/s on a 150Mb line.

Edit - changed my mind, sorry - it's looking like every 4s.
I was looking at the wrong info, my bad.