The drive from Plakias to Timpaki and Phaestos is very nice and the road is very good. But count on at least 90 minutes each way. We have done it every year since we split up our stay in Crete between Kamilari and Myrthios. In fact, Kamilari is very close to Phaestos and if you are into Greek restaurants, we recommend that you have lunch in restaurant Milonas. Very good authentic Greek food at very affordable prices. Kommos beach is very close to Phaestos as well and is one of the nicest beaches on Crete. Also nice to visit is Ligres beach, which has been described here on this forum in several other posts.
We are sure you will enjoy the trip.