How many years for us is a year in Narnia?

That depends. When the Pevensie children first traveled to Narnia in The Lion, the Witch and the Wardrobe they spent probably 10 or 15 years there, buy when they returned through the wardrobe it appeared that no time had passed at all in our world. Then, a year or so later when they returned to Narnia, they found that hundreds of years had passed there. The same thing happened in subsequent stories. So the answer is that time passes differently in Narnia, so its an "Apples and Oranges" kind of question.