Most people I have asked just knock it in for 60 minutes!
I start by rounding off the edges and getting rid of any 'sharp' corners! Then up and down the edges once they're rounded off. Next I concentrate around the toe area and the bottom 4 inches to strengthen that!
I do knock the rest of the blade in a bit but only really for 30 minutes or so! But the most important parts are the toe and edges to ensure the willow is compressed and strengthened!
Then I take the bat into the nets against the bowling machine for an hour or so, and progress onto throw downs and bowlers using soft cricket balls (as the leather is softer)
By this time, the bat should be pretty much ready to go.
The important thing is to realise that knocking in a bat is ensuring it's ready to play against fully new and hard cricket balls. But this is not to be confused with having the bat ready at it's optimum performance level, as this can take weeks or months, depending on the pressing, grains, how often you play, etc!