A long rest typically lasts for a full night of sleep.
Relaxed Setting
During a long rest in a relaxed setting, characters recover their maximum HP and maximum MP.
A relaxed setting can be an inn, a host's house, or any other stress-free place that offers comfortable beds and a good meal.
Stressful Setting
In a stressful or uncomfortable setting, the night's rest will be equivalent to 3 consecutive short rests.
These consecutive short rests do not count towards the 2 short rests per day, but it is still necessary to choose which characteristic (HP or MP) will be recovered during each short rest phase throughout the night.