My experience has been that the ECU basically adapts to your driving habits. This has a lot to do with open/closed loop tuning/response which I won't get into right now, but suffice to say, your car "learns" in a non-boost mode, and uses preset set maps once you are into boost.
So, if you drive aggresively after an ECU reset, for the period of time that you drive this way, the car will respond a bit quicker and retain it longer. The intial change would take anywhere from a few days to a few weeks. However, not everyone continuously accelerates hard into boost, every time, every shift, so eventually the ECU will adapt and the car looses some of the edge.
The only time I ever did this was once before a trip to the drag strip, and a few times for non-hp related items associated with maintenance or modifications. I kind of look at it as eventually, it settles back down to it's normal..."all-around" settings where it is good at everything (accel, passing, gas mileage, etc), but no longer has that slightley better kick in the pants feeling the reset gives you.
Also something to consider..the reset lasts about as long as the first set of knock events are recorded. Once that occurs, timing starts getting retarded and the ECU learns that whatever caused the knock to occur is a no-no, and will adjust accordingly. Typically, this will happen on hot days, or if you get a dose of bad gas. Which is why my thoughts have always been that if this is the case, then basically I am sitting on the edge when it comes to ECU tuning parameters every time I reset the ECU. So I ask myself, is this really worth it to me? Not really. Does it really hurt anything? Probably not, but as my daily driver and a father of 4, it just doesn't fit into my overall equation to even go there. For others...shouldn't be an issue at all.
Just so everyone is clear, these are my thoughts and experiences, and certainly should not be taken as gospel, hard-facts. Yours will certainly be different and you should approach how you drive and treat your car accordingly.