I've traveled this route more times than I can count. However, I don't understand your question. Are you planning a day trip to Dresden from Prague? (Which would be quite easy, as there are numerous train departures each day between both cities). Are you talking about the train ride itself? If so, it's about 3 hours in length and is probably the most scenic in Germany after the train ride along the Rhein between Mainz and Koblenz.