How many months will it take my hair to grow from neck length to shoulder length?

It's different for different people. For example, my hair used to grow pretty quick, but some reason, it's started growing really slowly. On average, I'd guess about 3-5 months for a noticeable difference, such as a little bit past your shoulders. Hair is made of protein, so maybe you could make it grow faster by eating more protein. I personally believe that, because around the time my hair started growing slower, I was lacking protein and iron pretty badly. I didn't mean to...I tried being a vegetarian for a few years, but I'm picky so I didn't have much to choose for substituting my meals and ended up eating a lot of junk food and empty calories. Way off topic, but anyway I don't recommend it.