I timed myself walking from the Makkasan airport link station to the Petchburi MRT station (exit 3). Platform to platform, walking at a brisk pace, was around 8.5 minutes - I did not have luggage. There were posted signs showing the way to and from either station. Pretty easy transfer.
Also, while standing at the platform at the Ram station waiting for the city link train, felt an awesome rush while the express train zoomed past. Man, those trains are fast - reminded me of the Narita NEX trains blasting through Chiba station.