On Codex Version 1.2.0.344, the time interval before the device goes to light sleep or powers off is stored in /home/root/.config/remarkable/xochitl.conf

  ...
  IdleSuspendDelay=1200000
  ...
  SuspendPowerOffDelay=43200000
  ...
  IdleToSuspendDelay=200000
 

IdleSuspendDelay → LightSleep IdleToSuspend → LightSleep to Suspend

If the device is just updated, the configuration lines might not exist. In that case, go to settings → power and disable then re-enable both Auto Sleep and Auto Power off. Both numbers are in milliseconds (i.e 1200000ms = 1200 seconds = 20 minutes). Restart the device (or the xochitl service) after changing the number.