Pic title: Don Muang Airport, Bangkok (Link)
Like the old Bangkok Airport, taxis are the best option between the airport and downtown. Estimate about 45 minutes from Sukhumvit to the airport - more if it's directly during rush hour.
If you've flown through Bangkok before, prepare to add 25% to previous estimates for both elapsed time and cost. Without traffic (late at night, early morning) it's taking ~ 30 min. to/from Sukhumvit/Asoke (Soi 21/Ratchadapisek) via the Bang Na-Trat Road, and the meter has been ~ 280 baht, + 40 for tolls.
From Don Muang, the travel time in late afternoon (4-ish) to Sukhumvit/Ploenchit hotels (Hyatt, Sheraton, IC) has taken anywhere from 45-75 mins in limo or taxi, with at least half the time in traffic once off the toll road.