There is no single lifespan for every perfume
Some bottles remain stable for years while others change sooner. A printed period or date can be useful, but it does not describe every opened bottle perfectly.
Smell is the most useful clue
If the opening becomes unusually sharp, flat or noticeably different, or a familiar part of the scent seems missing, the fragrance may have changed.
A color change does not always mean the perfume is ruined
Some fragrance materials naturally darken over time. Color alone is not enough, so consider the smell and storage history too.
Heat, light and air matter
Repeated heat and direct sunlight can speed up changes. A stable, moderate and relatively dark storage area is usually a better choice.
When should you stop using it?
If the scent has changed enough that you no longer enjoy it, or the liquid or sprayer shows an obvious problem, there is little reason to keep using it just because the bottle is not empty.