Typically, fire doors are required to have at least ¾ the rating of the wall/barrier in which theyre installed. For example, a two-hour wall is required to have a 90-minute rated door while a one-hour rated wall is required to have a 45-minute door.
The reason the door carries ¾ the rating of the wall/barrier is because combustibles such as boxes are not stacked in front of the door.
There are a few exceptions to this rule, such as when a hazardous storage area has more than one door and one of the doors is permanently locked and it is used for storage. The door must now be converted to a 60-minute door.
For more information check out this great article from iDigHardware.