Keep scents out of direct sunlight, and they will last a long, long time.
Perfume The Guide says they will last indefinitely if kept out of sunlight.
And one member posted here of finding his father's old colognes after 30 years of being under the kitchen sink, and that they all smelled okay to him.
Often their top notes change a bit with age, so that they may not smell as good for the first twenty minutes. But after that they become the same as the scent in a new bottle, since the mid and basenotes haven't changed.
I've never had any problem with bottles in my steamy bathroom - since they are sealed.
Which remindes me, there has been discussion here over the years about storing bottles upside down or on their sides - to keep their spray mechanisms from drying out, and then not maintaining a good seal. The jury is still out on that one.