I am not expert, but whenever i have knocked in a bat with and by that i mean preparing the bat rather than hiting it as hard as i can.
More has always been better.
To me you get to know the bats performance by knocking it in more and personally i have felt that you can expand the middle somewhat and definitely improve performance by long knocking it. I guess as mentioned there is a danger of creating too much of a surface hard layer, which i guess would decrease performance, but then someone else might argue that this larger harder surface would increase rigidity and rebound to hit it further, so....
I am guessing here, but i would have thought logic would suggest that better wood with more grains would need less knocking in and you should stop when the rebound off the mallets stops noticeably improving and seam marks stop being deep.
lessser grained harder bats i think you can really work them with a mallet into something much better than you originally bought
However saying all this it is freaking tedious and a noise my girlfriend unsurprisingly cant stand so...... give me one of those Lingums any day.
I trust myself not to ruin my bat.