I sit beside the fire and think
of how the world will be
when winter comes without a spring
that I shall ever see.
For still there are so many things
that I have never seen:
in every wood in every spring
there is a different green.
I wonder how many winters he would live for.
Duolingo isn't designed in a way that ensures you get everything right on the first try. Nonetheless, you don't need "to get this exact phrasing"; you just have to produce a proper translation. I wrote "How many winters long?", which was accepted as a correct translation because it means the same thing as "For how many winters?" (at least in some contexts).
Either way, don't forget that getting things wrong is a normal part of the learning process. When you use a foreign language, you inevitably make many mistakes; thankfully, finding out that you were wrong is a great way to learn and reinforce what's correct!
I think I was a bit peeved because, when I answered, "How many winters long?" was not an accepted answer. It's indeed true that often mistakes need to be made in order to improve, but I find that Duolingo is on the whole (at least, for the Dutch course) lacking in information on idiomatic expressions and phrasing. It's just a little criticism.