I'm with infrasilver with this - the knock sensor is part of the engine management system and therefore not 'learned' or adjusted over time.
Put it another way, if it took 'tankfulls' for the DME to adjust, putting inferior fuel in would lead to knocking for a tankful or so, and I have not experienced this. On trips through Europe it is often difficult to get 98ron fuel, so when I put in 95 E10, there is no knocking detected at all - though I do notice that the car runs at a slightly higher temperature.
So I think adjustment is immediate and continuous.