My teeth has been yellowish since I was a child. How long would it take for my teeth to become white?

Different shades. All teeth like people come in different shades and sizes. If your teeth have always been this shade and you are an adult, chances are they will remain like this. Talk to you dentist for available options on teeth whitening and if it is viable option for you.
Rapidly. Yellow and white are the normal color of teeth-white on the outside and yellow on the inside so they show normally whiter. Some people think they should show no yellow so they brush away their white color(enamel and expose the yellow (dentin). Some people are born with thinner enamel. Veneers, crowns and bleaching are ways to whiten teeth. See your dentist.