There have been a number of threads that discussed this question. If you search for them, you will find many answers different than mine.
Depending upon the fragrance, it takes me about 22 - 28 sprays to fill a 2ml atomizer. If I assume 25 sprays equals 2ml then 100ml equals 1,250 sprays. If I use four sprays per day, a 100ml bottle should last about 312 days.
100ml of Angel used to last me about a year, but since I can't smell it any more (I think it battered my olfactory senses into submission!) this bottle's been half way down for months while I explore other smells.
I wouldn't recommend wearing the same fragrance every day unless you fancy being unable to smell your favourite smell, like me.
For me, around half a year. Every day around 6-8 sprays throughout the day. Depending on the strength of the perfume I wouldn´t need to re apply and it would last longer with less sprays.
If you are considering a purchase, think how often you need to spray to smell it all day and use that as a baseline. It´s about 12-14 sprays to the ml, so you can do the math yourself to see how long this particular bottle would last.