The answer to 'when to replace skylight' is that skylights typically need to be replaced every 15-20 years, or sooner if they show signs of damage or leaks. Skylights are exposed to the elements and can deteriorate over time due to UV exposure, temperature changes, and weather conditions. Signs that a skylight needs replacement include cracked or cloudy glazing, leaks, and difficulty opening or closing the skylight.
Replacing a skylight is generally best left to professional roofing contractors who can ensure proper installation and sealing to prevent future leaks. Attempting a DIY skylight replacement can be risky and lead to further damage if not done correctly.