Why isn't "He sat for a long time by the window" acceptable as an answer here?
From the English side, it seems to me that it should be accepted, unless there's some word order thing going on on the Czech side. If you didn't already report it, you might do that if you get this exercise again.
If you enter a translation that's not accepted, and you believe it is correct, just click on the Report link and choose "My answer should be accepted." That will send your actual answer to the team, where it can be reviewed. FYI, it's much easier to report it when you're in the exercise, because there's no easy way to get back to it later.
If he above was accepted, I think this should be accepted too: "He sat a long time by the window."