When I am asked such a question "How many years experience do you have on the skill xxx"? in interviews, I don't know how to respond it correctly.

Because the culture difference, I am a humble man and don't sell me well. For example, I touched a skill "xxx" in year 2012, sometimes on, sometimes off.
When I was asked, my answer was try to calculate the sum of pure months of using the skills. So I always don't get opportunities. 

So tell me what is the correct answer?