I am travelling BKK to Dongcheng District at 7pm on a Sunday night. Should I Taxi?

We have only the night to visit the area and we are staying at Tianan Rega Hotel - Nan Heyan St - Dongcheng District. Is it easy to find a taxi at the airport? How long should it take in a taxi that time of the evening and how much should it cost? Thank you for you help.