It takes 4 days to walk to Finisterre via Muxia from Santiago - for a total distance of 115 km. (I am assuming you what to know the distance from Santiago and not somewhere else)
I just finished my second camino from Saint Jean PdP this past May and June, arriving in Santiago 30 days later (distance of 780 km). I then continued to Muxia (the less traveled way - 3 days) and then followed the coast to Finisterre (1 more day).
I would recommend going this way, since there are more bus option back to Santiago from Finisterre.
The route to Muxia is quiet and more like what the camino was before. The signage is good, but a bit confusing in some section. You will discover that along the way to Finisterre from Muxia that signs indicate the direction either to Muxia or to Finisterre - people travel both ways and you pass pilgrims going the other way.
It is typical Galician landscape and terrain, so expect anything including fog, rain and maybe cool days, even in the summer. But is probably the high light of the camino especially given the number of people walking the Frances route to Santiago.
Keep in mind that you will earn two more compostelas - one for Muxia and another for Finisterre. In order to receive the later if you arrive via Muxia, you must stop at the only cafe along the way and get a stamp.
There is some question about the distance between Muxia and Finisterre - some books suggest it is only 28 km, but we passed two signs one indicating it was 12 km back to Muxia and another indicating it was 19 km to Finisterre.
There is more pilgrim style accommodation along the way but more people walking this route, so throughout the day, you might find yourself alone but don't be surprised to find 100 plus pilgrims in the village for the night.
Check out the last 12 or so photos of my current set of photos for a feel of this special section:
https://picasaweb.google.com/1094336834 ... ntiago2011