What's the approximate shelf life of an eau de toilette such as Joop! or CK One once theyve been used for the first time?

As long as it's not exposed to heat or light it stays good for decades. There are perfumes from the 1920s that still smell good. It's mainly alcohol. If you want it to last longer, you can even keep it in the fridge

You should keep it out of sunlight and bright lights for it to last nearly forever.

Otherwise you're talking a couple of years as it's reaction to light that spoil perfume