The train timetable from Don Muang airport to Hua Lamphong train station is pretty useless, because the trains coming into Bangkok are usually running late because of track renewal or whatever.
But this does mean it's worth checking on the platform if there is going to be a train soon, as it's quite possible you may catch a late-running train that you would have missed if it had been on time. The train station is directly connected to the airport by a ramp and walkway over the eight lane highway, so it only takes a couple of minutes to reach the platform.
Trains take about 45/50 minutes to reach Hualamphong train station, which is about the same as a taxi would take in rush hour, and the fare is 5 baht. Also note that all the trains into Bangkok stop at Bang Sue, where there is a direct connection to the MRT subway line, which itself connects to the skytrain after two stops, at Chatuchak Park/Mo Chit.
And if you have to wait a few minutes there is a refreshment stall and other facilities on the station.
Going out to the airport, the timetable is fairly reliable, as the trains start their journey at Hua Lamphong train station and usually leave on time