The answer to 'when to replace skylight' is that skylights typically need to be replaced every 15-20 years, or sooner if they are damaged or leaking. Skylights are exposed to the elements and can deteriorate over time due to UV exposure, weather, and general wear and tear. Signs that a skylight needs replacement include leaks, cracks, fogging or condensation between the panes, and reduced energy efficiency.
Replacing a skylight is a significant investment, but it can improve the look, energy efficiency, and functionality of your home. It's best to have a professional roofing contractor inspect your skylight and provide recommendations for repair or replacement.