Stamina is an attribute that allows a certain character to remain active, exert itself to prolonged periods of time, and recover from ordeals.
We have no set categories for describing this characteristic as it is difficult to define stamina with great accuracy, rather, this is meant to be a guide on how give a general gist of how great ones stamina is.
It may also be a good idea to display feats of a character for example: "The official handbooks have described Thor's stamina as virtually inexhaustible. He once fought an army of frost giants for 9 months without tiring."
Additionally, this gives you the opportunity to flesh the character out more rather just writing "high" which is broad and could mean anything.
Below Unfit: Said characters have virtually no stamina at all and find it difficult to lift a finger.
Unfit: Such characters live an unhealthy lifestyle and find it taxing after completing basic tasks. Walking is difficult and exercise is near impossible to do without collapsing immediately.
Average: Comparable to the average human who does little exercise but is able to walk a mile or two before becoming exhausted.
Above Average: This kind of person able to jog reasonable distances before becoming tired. Are able to exercise on a regular basis and complete a full workout routine with some difficulty.
Athletic: Are able to compete in sports/marathons and are able to last the full duration of the game/course, albeit by breaking a sweat.
Peak Human: Can compete in sports/marathons and are the top of their game. This kinda person pushes themselves to the limit of their ability in every workout.
Superhuman: Marathons are considered basic workouts and are relatively easy to complete. Can probably skip the odd day of rest if necessary.
Godlike: Characters are able to stay active for days if not weeks on end without the need for rest or sleep.
Godly: These characters do not really need to sleep or rest at all and can stay active for months on end without needing rest.
Virtually Inexhaustible: Sleep is a very rare occasion and is probably only needed once a year in order to recover.
Nigh-Infinite: Sleeping isn't necessary if avoiding long lasting workouts. Theoretically no rest of any kind is needed as long as there is nothing serious to exhaust this type of person.
Infinite: This person doesn't need to rest or sleep full stop. This person may or may not be able to sleep, but can choose to if desired. This usually applies to mechanoids.
Irrelevant: This being transcends the need for rest, often due to their dimensional nature.