How long should it take to evacuate a building by NFPA standards?

It depends upon type and size of building, type and level of hazards, and other criteria. For example, a modern high-rise office would almost never be evacuated all at once, but an explosives factory may need additional fire exits for quick evacuation.

Some occupancies are rated according to how long it takes to evacuate, requiring additional life-safety protections for facilities having slow or impractical evacuation times.