I'm rereading Dune and I believe it's set about 20,000 years in the future. That adds wonderfully to the story for me.
Why? There are some
basic constraints, like not setting a novel about the heat death of the universe in the 2250s, or a novel about the first expedition to Alpha Centauri in 2020, but for most works the period could've been chosen by throwing darts at a spreadsheet. Many SF works, most famously 2001, depict futures that are now in our past. Doesn't mean much. Certainly there's nothing in Dune that makes 20,000 years a more or less credible number than 5,000 years or 50,000 years.