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.
Can it be "He have been sitting by the window for a long time"? Most sentences in the Slavic languages can mean either continious-type or simple-type action, and we only need context to understand it. In this case, for example, we do not have additional information (particular momnet in the past, which denotes a Past Simple sentence), but what we do have, is the adverd "dlouho", which makes me think this is a Continious action.