From Khao San Road it takes about 20 minutes by taxi in the morning and about 30 minutes in the afternoon. The taxi will charge on the meter between Bt. 45 tot Bt. 80 (€ 1 tot 2).
From Hua Lampong station it takes about 15 minutes by taxi in the morning and about 20 minutes in the afternoon. The taxi will charge on the meter between Bt. 40 tot Bt 60 (€ 1 tot 1,50)
This card in Thai language you can print out, and give to your taxidriver:

