Start shopping for replacements when the following problems crop up:
Small pockets of decay can be scraped out and filled, but if the sill and jambs are severely compromised, there's no saving them.
When the perimeter seal on a double-glazed window fails, the condensation that collects between the panes of glass spoils the view and reduces energy efficiency. The window needs a new sash, at the least, if not entire replacement.
Is a sash hard to open or will it not stay open? Those old windows from the weight-and-pulley era are easy to fix, but sashes with tubular-balance hardware from the 1950s, '60s, and '70s may not be worth saving.
A 19th-century Italianate fitted with modern replacement casements, for instance, will have infinitely more curb appeal with new wood double-hungs that match the look of the original windows.
Pictured: Rather than replace these 18th-century windows, TOH master carpenter Norm Abram restored them to like-new condition. Adding interior storms made them as weathertight as new units.