The 30% figure comes from one of the very few clinical studies on e-cigarette success rates. I won't call it 'quitting smoking' because that is a whole different debate - it is preferable, to many of us, to refer to the process as switching to an alternative.
Be that as it may, there isn't much evidence for any statements about the success rate. A telephone / web survey (by Polosa?) established that 31% of consumers who bought one particular brand were still using it (or another brand) after a given time. This being a mini, and no support having been available, you could justifiably say that there is some evidence for a minimum 31% success rate at [X] months, since this would apply to the worst possible set of circumstances.
With full access to any/all hardware needed by the individual, full access to any/all types/strengths of e-liquid, and full mentoring, it could be expected that the success percentage would at least double.
It would be necessary to carry out several trials, over some of the longer timescales certainly including the standard 20-month mark for final determination of smoking status, run by different researchers; then we would be able to say, "This is the evidence for e-cigarette success" - in terms of switching. Of course, none of that is necessary for the ordinary person, as they can simply look at the facts: millions of people using them, usage growing every year, very large and enthusiastic communities growing around their use, and vaping prevalence growing by about 50% a year. If it didn't work, none of that would exist.
However, medics are different from ordinary people and need to be able to demonstrate in research what can be seen to exist.