Well, it's like this, depending on your gear and the watts you vape at, drastically determines the amount of juice one vapes, plus how much you vape. When using beginner to intermediate gear and vaping between 6-8 watts, I probably only used 5-6 ml of juice a day. Now that I have several rba's and vape at 15-34 watts, depending on which pv I am using, I use more like 13-15ml a day, and sometimes up to 20ml on my days off. To offset my cost I now DIY juice so it's still pretty reasonable. Most vape more like 3-5ml a day. Like said depends on personal preferences, and gear and watts.
Heck, when I was using cig-a-likes, think the most I vaped was 3 ml a day.