How long your favorite scent lasts is up to you. Properly stored, every drop will still carry the same lovely aroma that greeted you when you first unsealed it. Scent bottles have been found in the pyramids that still carry a precious whisper of scent. However, like fine wine, perfume must be stored properly and used with care.
Do not refrigerate your scent. Perfume and cologne were designed to be stored at normal room temperature. This does not mean they should be stored near a heater vent, or in a draft. Choose an area with a stable temperature.
Do not keep scent in the medicine cabinet. Bathrooms can be hot and humid, which is not a good atmosphere for preserving scent. Incidentally, the medicine cabinet is not a good place for prescription drugs, either. They can also deteriorate.
Do not keep scent on your vanity. Choose somewhere dark, because light also degrades perfume. A closet or cupboard might be ideal. It you choose to keep your scent out, leave it in all the original packaging to protect it.
If you have a large bottle of scent, you might want to transfer a small amount into a purse applicator, and pack the rest away, tightly sealed. This way, you can apply a dab whenever you like, and still preserve the original aroma of your perfume.
Finally, keep you scent tightly closed. Air can contribute to its decay, and evaporation can cause it to thicken, while some of the volatile components can be lost, changing the balance of the composition.
In addition to light and heat, human touch can also degrade scent. Everyone's skin has oils. These contribute the chemistry that makes a scent our own. However, skin oils can also degrade a perfume, changing its composition. A touch may also introduce microscopic life that can hasten the scent's decay.
An atomizer or mister can help here, making it possible to apply scent without touching the bottle. If you don't like to use a spray, then use cotton or a bit of cloth to dab the scent on your pulse points, or at least wipe the rim of the bottle after use.
Sometimes the change in a cologne is obvious. It may thicken or darken as it loses volatile components or alcohol. Perfume that thickens is no longer the scent you bought, and you will know it. It may even carry a sour or cheesy smell.
Sometimes, though, changes can be more subtle, and you won't notice anything until you have worn your scent for an hour or two. Or, you may only notice that your scent has lost its intensity, dulled.
It is also possible to become "blind" to your own scent, and not even be able to tell whether it has changed. If this happens, perhaps you should choose a new scent instead of a new bottle of your old potion.
To keep your scent fresh, keep it at an even temperature, tightly sealed, and away from light. Keep it pure, to keep it longer. Replace it when you notice it getting darker or thicker, or it the scent sours or fades. If you become unable to smell your own scent, try another one for a while.
Perfume is one of life's most delightful luxuries, and well worth treating with care.