For questions of Earth\humans that extrapolate beyond available data: hundreds of years at least, thousands commonly, millions preferably.
Predicting the future is dicy practice at best, but "far-future" questions ask us to imagine a world way beyond the current day. These questions look at the Earth\human\solar condition on geologic and cosmologic timescales.
Far-future questions run the danger of being closed as "primarily opinion based." None of us knows what is coming (or, if some of us do, they aren't sharing the oracle technology with the rest of us). Make your question as specific as you can. Let the community know what parts of the present you consider as fixed points. For example, when you ask about humans building our first Dyson Sphere a million years from now, let us know whether humans are still humans as we know them today or whether we're all uploaded minds controlling cyborg bodies. It makes a difference!