My flight arrives midday at 12.05 on a wednesday. Im will only have carry on luggage and I have a fast track service through.the.airport with taxi booked 

Depends a lot on whether ‘12.05’ is midnight or noon, as well as where within Bangkok you want to go … it’s a large city. 
 

At midnight, taxi takes about 30 minutes to lower Sukhumvit.  At mid-day, I wouldn’t bother with the taxi due to traffic, but rather take the airport rail link and MRT/BTS - about 45 minutes to lower Sukhumvit.  
 

16 minutes ago, tobeornottobe said:

My flight arrives midday at 12.05 on a wednesday. Im will only have carry on luggage and I have a fast track service through.the.airport with taxi booked 

I will say 30 minutes to go outside airport. After it depends where you go. 

12.05  is five past midday  . @momo5

I said midday in my post. Staying in the Ambassader on soi 11

Tell the driver to leave the expressway at suk , soi 50  , that's at On Nut coming in at the side of Lotus supermarket  . I have always found this the best way . By 13.00 you should be at your hotel