My experience of GM having used them mainly for the last 3 seasons is the GM now means they are a little hard, now personally I don’t use a mallet to knock in bats. I have found that straight into nets against Bola balls works perfectly given about 5/7 sessions at about 30 mins each. Even better would be if you had nets where you could face spin or slow bowling with quality d league balls. This way you get used to pick up and feel as well as bat prep.
Anyway about 5/6 hours I have always found and then good to go, I open the batting and always fave new ball and never had any issues with damage or performance after my bat prep.