I am not an English native, but is "Ten years are a long time" really wrong (as I was told when I entered this as my solution)?
"Ten years" refers to one period of time, so it takes a singular verb.
Why not "ten years are long"?
That would be "Ti år er lange"
"længe" means "a long time"
As jungerstein stated, 'ten years' is a single period of time such as a decade, and will take a singular verb of 'is'.
Would it happen the same with eleven years?
Yes. This because time is a singular noun; 'are' for plural nouns, and 'is' for singular nouns.
I wrote "are" instead if "is". Even if, as some say here, "is" is correct, I think as a correct translation "are" would be also sufficient and should be accepted. At least this is a Danish course and not English ;)