The answer to 'when to replace fascia' is that you should replace your fascia boards when they show signs of significant wear and damage, such as cracking, warping, or rotting. Fascia boards are the horizontal boards that run along the edge of your roof, and they play an important role in protecting your home from water damage.
Over time, fascia boards can deteriorate due to exposure to the elements, and it's important to replace them before they start to compromise the integrity of your roof. Generally, fascia boards should be inspected every few years, and replaced every 15-20 years or when they show signs of significant damage.