I vape around 5-7mL of 18mg per day. Sometimes more, sometimes less, it depends quite a bit on the device I'm using as well.
Since I began solely using RBA's/RDA's my juice consumption increased quite a bit. Although, it only takes 2-3 pulls to satisfy my cravings now lol.
I wouldn't worry too much, hell, I've seen users on ECF claim more than 30mL a day!