Hit Point recovery occurs organically with a full night's rest of 6 or more hours.

The amount recovered depends on the conditions of rest:

Unsafe Rest (cavern, dungeon, camp, etc.):

Recover 25% of your maximum HP after a full night.

Safe Rest (inn, stronghold, etc.):

Recover 25% of your maximum HP + (your Constitution Bonus x character level) after a full night.

Downtime (one full week without adventuring):

Recover all lost HP.