|Open shelves might not be the best choice to store your fragrances|
All fragrances consist of a complex mixture of organic chemicals and over time some of those will react with each other, with the ethanol they are dissolved in and with oxygen from the air. As a result of those reactions you can sometimes get a precipitate forming in the bottle, more often you’ll get a change of colour (usually going first dark yellow and then brown as time goes on) and a change in the scent. The scent change isn’t always for the worse - most fragrances improve for several days, weeks and even months after they are blended - but top-notes do tend to fade over time.
Fragrances like Chanel Number 5, that contain a lot of aldehydes, or those with a heavy dose of citrus tend to be the most vulnerable to deterioration as the aldehydes can oxidise and also react with methyl compounds in the fragrance (such as methyl anthranilate for example, a common perfume ingredient in it’s own right and also a common constituent of many natural ingredients) forming what is called a Schiff’s Base - the result is generally a softening of the aldehyde element of the fragrance which becomes less bright, sharp and more lasting. That can be good or bad depending on the fragrance and the degree, but it will certainly be different.
The hotter the fragrance, the more light, especially UV light, the faster all that happens. The colder, darker and the less exposure to air (i.e. full, sealed bottles) then the slower it is. Unopened fragrances were recovered from the wreck of the Titanic after nearly 100 years and they were still in excellent condition: but then it’s extremely cold and dark down there . . .
To give you an idea of the rate of change, here is the formula usually used as a guide for the influence of temperature on the ageing rate:
12 months at 20 degrees = 12 weeks at 40 degrees = 6 weeks at 50 degrees = 3 weeks at 60 degrees (Celsius in all cases).
That’s based on something called the Arrhenius equation, which states that the rate of reaction doubles for every 10 degree rise in temperature. There is some doubt about whether this is still true for organic chemistry once you go above body heat. So in practice perfumery companies test at about 0-4, 20-25 and 37 for 12 weeks or so to establish a perfume is stable under all the conditions likely to be encountered in most homes or in transit. But from the above you can see that your fragrances will age much faster when warm than when chilled and that escalates with tropical temperatures.
If they are kept in good conditions, fragrances can last for years, decades even centuries - as for example those recovered from the wreck of the Titanic.
Finally there is a widespread idea that the alcohol in a perfume can 'go off' and turn to vinegar - this is a myth - it simply does not happen. If a perfume does smell bad, it's likely to be because one of the aromatic ingredients in the perfume itself has oxidised or otherwise deteriorated. Fragrances containing Grapefruit oil for example can degrade to produce sulphur compounds, resulting in that characteristic 'bad eggs' smell appearing. For this reason most grapefruit fragrances are made with synthetic substitutes that don't include the sulphur element.