Taxi from Suvarnabhumi to Nana, should be around 40 minutes but can take from experience E few hours.more than that in traffic.
As was said above, take the train and BTS. Total travel time is usually around an hour and the cost is about 80 baht.
Getting to Don Muang taxi really is the only convenient way and unless you leaving in peak hour travel time journey is usually 45minutes to an hour.