When I am asked such a question "How many years experience do you have in skill X?" 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 used a skill X on and off for the last 5 years. 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 what is the correct answer?