One of the key moments when you are having your olives pressed at the Frantoio is when they tell what your resa is. This is the yield of oil in kilos compared to the weight of olives you have had pressed. This can be a matter of pride or of quiet reflection if it is low.
I have had yields from 13 to 18% which reflects the different times of picking and pressing. The higher yields are from the most recent pressing when the fruit was all ripe.
Other regions will have very different yield levels. I was speaking to someone from Tuscany the other day and he spoke of yields around 24% being more the norm.