The Airport Express is supposed to take 15 minutes for the non-stop journey between SBIA (BKK) and the CAT at Makasan. This train is supposed to run every 30 minutes but from all announcements, in the early stages this will hourly service, so a departure every 60 minutes. Most expect this line to operate ~ 06:00 to 23:59 daily. When City Line service begins that journey will take 30 minutes SBIA to CAT, and most expect service every 15 minutes. Express trains overtake City Line trains at intermediate points or stations.
Obviously, until these trains are in regularly scheduled operation it is impossible to ascertain their usefulness. By far the biggest problem is arriving at the CAT and then dealing with onward transportation. Traffic in this area is usally bad; yesterday I sat in a taxi for ~ 45 minutes on Asoke/Ratchadapisek in front of the CAT. Or one can walk to the Petchaburi MRT station (until some sort of connector is constructed this will be a very, very long, and challenging walk), or travel onwards on the City Line to Ratchaparob and Phaya Thai, once City Line service commences.
I might take the train but I certainly would not want to wait more than 15 minutes for the Express train. Who knows what the taxi situation will be like at the CAT?