How often should I replace my mask and fins?

On condition, not calendar. Masks retire when the silicone skirt hardens, yellows, or stops sealing — years of UV and heat exposure decide, so store them dark and cool. Fins last a decade-plus unless straps or buckles fail (replace those, not the fins). The realistic trigger is fit drift: faces and feet change, seals stop sealing, and no product fault is involved.