You are right, it does check continuously, but there is also a point at the 10 day mark where BT record the line speed you have achieved and that is then used by them as a benchmark of what is a stable speed for your line if you have to complain of reduced lines speeds later.

http://aaisp.net.uk/maxmagic.html

So I was wrong, there isn't some magic 10 day period at which point the line monitoring changes, but the 10 day mark is still significant.