Start your timer when you get the first fire warning.
Lower belly means you have fire bottles at your disposal. Use them, while diverting to the nearest adequate airport at the highest speed you dare.
I wouldn't descend below FL250 because you might need thin air.
The moment you have confirmation that the fire is real: ditch or make an off-airport landing within the infamous average 17 minutes of survival time...
Don't worry about the airframe; the moment a real fire breaks out it's a write off anyway.
Worry about one thing only; the souls on board.
And good luck, you'll need it.