From what I remember of reading the manual, 95octane petrol will cause knocking noises to occur until the knocking sensor is activated, or something along those lines.
But a couple of comments, firstly, 'knocking' will only ocuur when the engine is loaded (fairly hard acceleration, with a wide throttle opening), and will NOT occur at tick-over/idle. Secondly, true knocking will only be intermittant, happening for very short periods - as soon as the sensors detect knocking, the ECU very quickly (instantaneously) retards the ignition timing - and so it really shouldn't be audible at all.
No idea how long this would take.
The response to actual knocking, as it occurs, is immediate. But for the ECU to 'permanently' adapt to lower octane fuel can take between 3 and 4 full 'duty cycles' of the engine.