It depends entirely on the composition of the existing door and frame. A fire door is not just the door, it is a way of closing an entire opening to stop or delay the passage of flame and smoke. The time varies with fire doors from 30 mins protection to 1 hour protection in domestic situations and sometimes these doors are simply solid timber but with intumiscent strips around them which expand in the heat to stop smoke getting through. Without knowing exactly how the existing door and frame are constructed it's not possible to advise properly. Sorry.
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