It is about 5Kms and some of it up hill. I am not sure but suspect about an hour from the old quay in Plakias, certainly more than 40mins.
It really depends on where you are staying, not a bad walk from the Labyrinth studios for example.
Personally would rather walk to Souda beach or up to Mirthios/ Myrthios.
Stick your thumb out and get a lift. We give lifts all the time, it is a Greek way of life in rural areas. Take a torch for the return journey, good to be as visible as possible even if you can see.
Good luck.