Is stream entry in 20 or 30 years of practice realistic?
It can be difficult to give an exact timescale since there are different factors involved in the practice, for each being. 

Some beings might have done extensive and concentrated practice in past lives allowing them to reach stream entry quickly in this life or future lives. 

Other beings might not have accumulated that kind of practice. All beings have different kamma. 

It might be dangerous to put a timescale on the achievement of stream entry since it takes one out of the present moment and into the future. It can also build expectations to the practice which can hinder ones progress. 

One might not solve all problems in this life or the next one but by practicing diligently one is altering ones habits towards goodness and these habits is what eventually leads us to enlightenment. If one does good things and practice diligently then the mind is inclined towards Nibbana. This means that when one is on the path it will eventually lead to liberation. Don't worry too much about when it's going to happen.