I have to agree with the amount you use having to do with the PV used.
I was a 4pack a day smoker and have been analog free for about a week.
I started with a 901 and was going through about 1ml a day
Now I have a Riva and noticed that OMG more vapor = more juice used, so vaping about the same as my 901 but with more vapor from my Riva I use 2-3 ml a day.
When I forget to order juice and realize I am low I switch back to my 901 until my order arrives in order to conserve juice.