Ideally you should knock in any new cricket bat for at least six hours, yes it's a lot but it will help condition your bat for heavy usage without damaging it.

You should also knock-in your new cricket bat even if it comes advertised as 'ready to play', as it still won't be ready for the full force of a hard new cricket ball, especially if you catch an edge or the ball hits the toe.

I've seen players go out on to the square with a brand new cricket bat, which hadn't been knocked in. In the first over he dug out a Yorker, which resulted in a 'fatal' crack in the toe of the bat. If it had been knocked in properly it's unlikely this would have ever happened. If you don't knock in your bat you could literally be 'throwing