It depends. It various greatly from one person to the next. I have friends who vape 3ml a day but others who vape over 20 a day. My husband vapes around 10ml a day. I vape around 15ml or more a day but I'm a chain vapor. One thing that impacts how much liquid you use is equipment. If you are sub-ohming you will use way more juice than someone using a pro tank or aspire nautilus tank. I use subtanks, RDAs, and many other devices so I use a lot of juice.
Smoking and vaping are way different. You can't really say if you smoked X number of cigarettes you'll vape this much liquid. It's dependent on the user, equipment, and other factors.