Hi Chyan, welcome to basenotes!
l can't speak for others, but l count it from the time of application to the time when l can no longer detect any trace of scent on my skin. Of course, the projection has usually lessened considerably long before that, & the time that takes varies a lot between different fragrances, & on different people's skin. On me, it's unusual to get longevity of more than 5-6 hours on any fragrance, & a lot of them settle close to the skin within a couple of hours. That's why l love so-called "sillage monsters" like Alien & Fracas!