It’s controlled by the Power Panel software. Edit/Show the profiles and look at whichever battery profile you’re using. You’ll see any entry for Standby Timer and Hibernate Timer. You can adjust either accordingly.
I wouldn’t worry too much about the Hibernation since it does it pretty quickly (and even faster if you have less memory) unless you’re violently moving around. You have to remember that notebook hard disks are designed for great shock and reliability than standard desktop drives.