The time to replace your windows is after they have lost their ability to insulate with a broken weather barrier (seal). Typically you will see what looks like condensation at the bottom of the pane, which means moisture is getting i between the panes. Once this occurs, a new panel of glass is needed. Assuming the frames are in relatively good condition, a replacement panel could be installed rather than a whole window, which is cost effective. If your purchasing new windows, pay extra attention to the warranty offered on broken seals. You will be glad you did.