It's a forty minute drive depending on traffic. They usually leave every half hour. Take the number 18,13,11,5, or 3 and get off at Alexandras Ave, just beyond the Archeology museum. You will see the park and the buses one block up. In Rafina the bus will let you off in the harbor.
Traffic can be heavy at certain times.
Going there on weekends can be hell. Go during the week and leave Athens early on days when the stores close at 2. The temple at Sounion was the first and last sign of 'civilization' Athenian sailors would see to and from their journeys. It's a magnificent site on top of a hill on the tip of the Attika peninsula. Not a bad place to watch the sunset either after a late lunch at a beach side taverna.