How long does perfume usually last ?

A chemical usually changes odor either because bacteria consume it, or it oxidizes (combines with oxygen) to become a different chemical. Perfume probably doesn’t appeal to most bacteria so it won’t change that way. And if it’s kept in a sealed container, it will take a very long time to oxidize or won’t oxidize at all. I imagine your perfume will remain sweet-smelling for quite a long time :-)

As long as you and anyone you are wearing it for like it, it doesn’t matter what the rest of us think about how it smells.

I agree with @WestRiverrat

It’s oils and alcohol. The oils stay the same, and if you keep the cap on tight, the alcohol stays in the bottle. Like Scotch or vodka.

I don’t think they change, you can have them for years and they seem to stay the same. How many things ordinary everyday things can you say that about?

Oils spoil, especially if not stored properly (exposed to light, air-etc). Perfume can last years if it’s mostly alcohol based. If it’s mainly oil based, not as long.