This depends on several factors, mostly the concentration of liquid nicotine in the eJuice product that you have selected. On average, one 30ml bottle of e-liquid is the equivalent to 2.5 cartons of cigarettes. This really depends on how much you vape and the voltage of your ecigarette or personal vaporizer battery. For example, if you smoke full strength cigarettes, each cigarette contains approximately 1.2mg of nicotine.
If you smoke one pack or 20 cigarettes per day, that would add up to 24mg of nicotine per day. That is equivalent to vaping about 1.33ml of e-liquid per day, meaning a 30ml bottle of 18mg nicotine strength would last you about 3-4 weeks. The average experienced vaper considers 1ml of eJuice to equal approximately 7 cigarettes.