I vape between 1ml and 4ml a day. This is depending on what my set up is, where I am, what I am doing and what mood I am in. I tend to have more than one device on the go, too.
In my experience, higher voltages often equate to higher consumption of juice by the device - but this is also dependent upon the juice. Some juices seem to last longer no matter how hard I try to finish that last bit off, at any of my available voltages. I've found this recently with Virgin Vapors.
An example, if your friend has a small cigarette style battery such as a Bloog and small standard resistance cartomizers and you have a variable voltage eGo Twist with a low resistance Kanger bottom coil or a dual coil then you will definitely go through more juice while vaping at the same speed/frequency.
YMMV but as they say, whatever keeps you off the analogs.