The general guideline is 2.5-3 hrs between Hua Hin and the city center, assuming moderate traffic conditions, on an "ordinary" day, and during non-rush hours in the city. Personally, even with our fairly assertive and somewhat lead-footed private driver, we've never made it in less than three.

Hua Hin<>BKK is different. Take your guidance from the Hua Hin Airport bus website, which says, "The estimate travel time between Hua Hin and Suvanabhumi Airport will be 3-4 hours subject to traffic condition" and goes on to warn that passengers should add time to their trip to allow for traffic and delays.

You will sometimes make better time than the estimates when traveling by road, but the chances of encountering a variety of issues and delays are much too high to ignore, especially if the greater Bangkok metropolitan area is involved. Plan accordingly.

Good luck and enjoy your visit!