Winter has been coming for five seasons lol, just how long does it take to come?
If I remember the book correctly, "winter" isn't a something that lasts 1/4 of a year but may last many years. Similarly, their "summer" may last multiple years (I think they referenced at some point that the current summer was the longest they can remember - maybe 10/15 years? - meaning that they expect a long winter as well). So when they say "winter is coming" they don't necessarily mean it's coming in a few months.
I think Bran has never seen winter during his lifetime, which is why Old Nan calls him "sweet summer child". That says a lot about how long this particular summer has lasted. I fully expect winter to hit the North this season, based on the hints that have been dropped by Stannis lately. I believe the first snows have even hit King's Landing at this point in the books, though I might be remembering wrong.