1) A sure sign it is time to replace your windows is if they are warped, cracked, or broken. If it is a minor problem such as new weather stripping then repairing the window might be a cheaper option. But if it is a more serious issue it is recommended to replace the window rather than throwing money down the drain to costly repairs. Signs to look for are windows that fog up, are drafty, stick when trying to open, and refuse to stay open.
2) Reducing your energy bill has a large part with replacing your windows. Drafty windows in the window can cause your energy bills to be 10% to 25% higher (energy.gov). Replacing your windows with energy efficient options can help your energy savings in both summer and winter. It is also a big selling point when listing your home for sale.
3) Does your home need a makeover? New windows are an option you will want to consider. As mentioned above replacing your windows can be a big selling point for your home. When windows look worn it can hurt the curb appeal of your home, consider a design that will fit the design of your home. Don't be afraid to add character while choosing windows that are also efficient.
4) A mighty storm just came through, or you live in an area with extreme elements. For example living near the coast, your windows will need to weather through severe storms, sea salt, humidity and high winds. If you worry about the integrity of your windows, consider Ultrex fiberglass which is designed to resist high winds and flex with extreme temperatures. These windows are also very low maintenance once installed.
5) It's history in the making, aka you have a historic home. You will want windows that match the exact design of the home. In this case, many companies specialize in matching windows with the historic character of your home.
If any of these cases match your home and windows, then it may be time to consider replacing them. After all, our windows are the eyes to the house or something like that!