Depending where you are going in the city, and the time of day, it can take you anywhere from 30 minutes to over an hour.
A taxi from the taxi que outside will cost about bt300. That includes the road tolls, and the bt50 airport charge. The taxis use the meter, but the bt50 isn't included in the fare you will see, so don't be suprised by the extra bt50 added to the charge. Also you will pay the road tolls as you come to them. There are 2. Give the driver a bt100 note, and he will give you back the change.

Hope this helps

koolbreez