In April this year I walked SDC to Muxia in 4 days: Negreira, Mt Alto, Dumbria and Muxia. 4 days is nice ... not especially challenging. I did this as a stand-alone Camino though, and it was more than enough for some of my group.
After my Camino del Norte this year, I went again. SDC to Muxia as above, using the same 4 day route. I then took two days to Finesterre with a stop at Lires, one day would have been enough really. We then took four days to walk back too.
Doing this in 7 days would be a bit much I think. Walking back was a bit of a drag too ... after 47 days walking with the pilgrims, I didnt much enjoy walking against the tide per se.
Some recommended stops:
Day 1 Negreira - Albergue Bergando - About <1km outside of town, but its really nice. Log cabin style on a kind of farm. A lot of animals and horses there.
Day 2 Mazaricos - Monte Aro - this is roughly 27km from Negreira. A really really nice place with great food too. Fairly new.
Day 3 Dumbria - The municipal is functional, but this is then an easy 17km day.
Day 4 Muxia - I've stayed in 2 places here, the municipal is quite nice and has a terrace upstairs. If you are lucky you can catch a sunset here.
Finesterre - Albergue Le Paz, I like the location of this one ... it's the last one before the lighthouse I think. It's quite a basic hostel, but being closer to the lighthouse works for me.
