If you have idp and driving license from your home country you don’t have to watch the video I’m pretty sure. But for bike and car with no license in home country it was 5 hours learning, then go back next day for test and collecting license.
This was at krabi transport office, I’m sure I’ve seen people say it varies depending where you go
If you come to Koh lanta via krabi anyway I’d do the extension on the way here. Saves you a trip off the island (which can take most of day with the ferries and such.)
You can do the extension at any point with less than 30 days left on your current stamp.