I vape with various types of devices, so it is dependent upon the device I am using and wattage will determine my juice consumption in a day.
Mouth To Lung at about 8 to 10watts in the 1.4 to 1.8ohm range - I chain vape on these all day most times, average about 5 to 8ml of 12mg juice a day.
Sub-ohm tank direct to lung in the 20 to 30watts 0.5ohm range - I can go through about 10 to 15ml of 6mg juice a day on one of these
Super-sub-ohm dripping atomizer direct to lung in the 40 to 80watts 0.25 to 0.3ohm range - I can easily burn through about 30ml of 3mg on these a day.
So as you can see just from my usage, style of vaping and the watts involved can burn down juice rapidly, I've known some super-sub-ohm level vapers that can vape almost 60ml of in a day, generally in the 0mg to 1.5mg nic level of juice. Due to the fact that Nicotine does constrict muscles and arteries, you have to balance the fact that the Nic content in juices is considerably lower than a single cigarette, so even chain vaping your actual nic intake in a day is far below what you were getting in a whole pack of cigarettes. If you were over-nicotine exposing yourself, you'd get nausea, headaches, and heat flashes first before going over the critical level of nic-poisoning yourself. That is also not taking into account the MOAI's, tar, carbon-dioxide, the acetone and other chemicals in cigarette smoke that compound and contribute to heart disease are not found in any vape out there or does not have the concentrations of them that a cig has (diacetyl, formaldehydes, etc).
1 Cigarette - after combustion on average you get a nic exposure of 48 to 56mg of the nicotine in the unlit cigarette
15ml X 6mg = 90mg total in your day
48mg X 20 Cigarettes in a day = 960mg total in your day
Sorry for the math, but...