Having taken the bus several times, it leaves on time and always arrives in under an hour. Suppose you get stuck in a long immigration line at DMK and you get on a bus just after six, ninety minutes after arrival. Let's also suppose it is the longest ride ever, 90 minutes, which is over 30 minutes longer than it's ever been for me. You still are at the airport 1:45 before your flight. That's more than enough time. Reminder: Check your airline's baggage cut-off time for BKK. I've never flown an airline out of BKK with a two hour cutoff.

A couple pointers: Have your ticket out of BKK PRINTED ahead of time. Printed works best, and you will need to show it to take the free bus. If you are running late at BKK, if you are not already eligible ask the people checking you in if they can give you a card for the faster security/immigration check. These are given to airline elites and business class, and can shave time off getting through security.