Stamina is an attribute that allows a certain character to remain active, exert itself to prolonged periods of time, and recover from ordeals.
Take note that we have no set categories for describing this attribute, and it is very hard to gauge properly, so you should preferably rather describe the stated or displayed extent for a certain character.
For example: "Thor once fought an army of frost giants for 9 months without tiring."
Infinite stamina should only apply to characters that can use their abilities any number of times without exhaustion.
Please note that characters that are mechanical or undead should not automatically be assumed to possess infinite stamina, unless this trait has been clearly defined within the series or by the lore.