I vape about 5 mls a day so technically 3 days but I vape so many different flavors that a 15 ml bottle could last me a while.
Chain vaping 15mls takes me a little over a week to go through. I vape at around 3.7 volts and only use cartomizers which eat up less juice than tanks or clearos.
Tanks use cartos .. or maybe your using zen style?