I think it's easier to work not based on the number of dabs but on the level of the juice in the vial.
1 ml = approx 10 to 15 sprays. Let's go with 10 for easy math.
So, visually divide the vial into ten segments, and apply one segments worth of scent to the skin, and you should have applied about 1 spray's worth.
Most vials aren't actually full to the top (they can't be) and actually are more like 0.7ml, but even so, this method is fairly accurate. In other words, if you normally wear one spray of a scent, you should get about 7-10 wearings from a vial. If you use the equivalent of two sprays, you should get 3-5 or so wearings per vial.
Of course, if the vial is larger than 1ml, as some samples are, you'll have to tweak the math accordingly. At 10 sprays per ml, a 1.5ml vial would give you approx 15 sprays worth, so then you'd - if you were trying to apply only one sprays worth - use only 1/15th of the vial.