Is stream entry in 20 or 30 years of practice realistic? The founder of the Triratna Buddhist Community, Sangharaskshita, stated that he thinks stream entry is realistic after twenty or thirty years


  In fact, it would be surprising if, after 20 or 30 years of life in
  the Order, you were not a Stream Entrant. It would seem to be very
  surprising, if you have done all the other things - maybe you have
  founded Centres and written books and given so many lectures and gone
  on so many retreats, that you were not a Stream Entrant: why not?


It's obviously a very bold statement. Does anyone else give that kind of timescale for stream entry or do any modern traditions give any kind of timescale at all. Or is Sangharaskshita a complete outlier with this kind of estimation.

Note: Tiratna is my Sangha which I think is right to acknowledge in the question. That said it doesn't stop me finding some of Sangharaskshita's statements bold or even controversial and I'm always interested on outside perspectives on his statements.
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.