I stuck with it for 5 years because I was paranoid about relapse to cigarettes. Spent a couple of weeks in countries where vaping is illegal as a dry run, earlier this year, and then just waited for the juice to run out.

The addictive effect of nicotine seems much enhanced by the addition of a monoamine oxidase compound (ie related to the early antidepressant drugs) - which is present in burning tobacco - which seems to take nicotine from approximately as addictive as caffeine to more addictive than heroin (citation available).