The best time for window replacement depends on the quality and state of your existing double hung windows. Generally, double hung windows made of lower-quality materials will only last about 7 or so years. High-quality windows can last for well over 15 to 20 years.
Some signs you should consider double hung window replacement include:
Drafts by your windows
Rising energy bills
Cracked or chipped window frames
Broken or cracked window glass
Condensation buildup within window glass
Difficulty operating or fully closing windows