Gravity is updated weekly on Sunday, between 3 am and 5 am your local time, at a random time installed as a cron script during install. The weekly interval is chosen with regard to overloading the blocklist servers, and considering the number of changes made to blacklists in a given week. Here is an example script that runs at 3:09 am local.
-rw-r--r-- 1 root root 1703 Sep 15 18:12 /etc/cron.d/pihole
9 3 * * 7 root PATH="$PATH:/usr/local/bin/" pihole updateGravity >/var/log/pihole_updateGravity.log || cat /var/log/pihole_updateGravity.log
If you change your blocklists from the web admin GUI, the "save and update" selection box updates gravity to incorporate your current set of lists.