How long does it take to become good in Yoga?

I just started yoga 3 days ago and I realize I am not very flexible and my arms are still not strong enough to support some positions that require all the weight of the body to go in the arms…I was wondering if with time it will improve and my body will become more flexible and therefore be able to do some of the hardest positions? How long does it take approximately if I practice every day for two hours….For how long should one practice in a day?