I pondered the OP's question throughout my weekend. I am, by no means, a stream-winner yet. I am still finding balance in the "right" view regarding this and from the little reading from the sutras I am convinced that stream-winning is possible. How common is it though? One extreme, assuming its a possibility, is rarity and the other is often; although I think it to be easy to say "rare" due to my relative view and apprehension that if it was so common "what am I doing wrong?" Finding a middle-way between rare and often would be uncommon.
This is likely a relative question that I find more interesting, is the attainment, dualistically speaking, "hard" or does our relative state create that delusion? What holds us back from it? (I tend to think clinging / passions, self-preservation, and doubt are it).
In the end, I am optimistic, be it metaphorical or literal, that if they say it can be done in seven days that it may likely be an uncommon and very real, achievable aspiration. And I really tend to think that it comes down to our level of devotion to the practice. A half-assed commitment, though meritous, is not enough. Excuses won't take us there, nor will zeal without wisdom. The wise and dumb are capable.
How convinced are we in the dhamma? And it cycles back to "some suffer too much, some suffer too little".
Food for thought anyways,