If you don't have a lot of luggage take the suburban train ("Proastiakos") and change to the metro. This is the only way to avoid getting stuck in traffic (which isn't bad near the airport, but, once you're out of the airport motorway, it can take ages...).

If you do have a lot of luggage, take a taxi and ensure the meter is switched on (tariff no. 1). You should have to pay around 30 Euros...Note the numberplate just in case...sometimes the meters get tampered with! If there's no traffic, you'll be there in under half an hour...Normal traffic, probably just under an hour...If there's a traffic jam all bets are off...worst case scenario I'd guess a couple of hours...