Personally, I wouldn't recommend this day trip but if one must....
Get either the:
08:15 EVA Transportes bus from Lisbon Sete Rios Bus Station arriving Lagoa at 11:15.
05:30/07:30 Renex.pt buses from Lisbon Oriente Bus Station arriving Armacao da Pera (Get off at the second stop by the sea front) at 08:50/10:50 and Lagoa at 09:00/11:00.
From either Adp or Lagoa get a taxi to Benagil. MAKE sure to arrange a time for a taxi to pick one up IN PLENTY OF TIME coming back or one is in for a long walk 6 to 7km walk back in the heat of the Algarve sun.
There are currently departures back to Lisbon from Lagoa at:
EVA/Rede Expressos. (www.rede-expressos.pt)
Be aware that there may be more bus frequencies in summer and times may change.