Lots of good info provided here already, but I'll just put my two cents worth of thoughts in...
In 2014 I walked the Salvador/Primitivo/Muxia/Finisterre during late February/early March, in about three weeks. Really. Multiple 50km+ days, and two short trips by car (where kindly old men offered to drive me down a mountainous path during heavy storms; I never sought help but they offered). As you are going in mid-March you might have more lenient conditions than I did...maybe.
Weather/Conditions: It was generally -5 to +10 degrees Celsius from Leon to Oviedo to Fonsagrada (the first town after crossing into Galicia). After that the weather warmed up and cleared up considerably, up to 20 degrees C and sunny all the time at the coast. Weather was anything from sunny and windy to blizzards and thunderstorms. On the Salvador I had up to waist deep snow in certain sections, and in other places in the mountains I was walking through ankle-deep water (from snowmelt). At the highest points the path was covered in snow which was covered in a layer of ice, creating dangerous conditions where a slip could mean a slide down hundreds of metres. Also, mud everywhere.
Albergues: (Salvador) They were open (some by request). I had counted on the family restaurant in Poladura de la Tercia to be open, but it wasn't, so I went relatively hungry on that night. I had to ask for the albergue in La Robla to be opened for me. (Primitivo) This was tricky, because some were unlocked but unmanned. One albergue in Fonsagrada was closed and locked, and I had to get a local to phone the hospitalero to come open it. In general I was lucky to be able to access the albergues, and if they were locked then I may have had a much more unpleasant experience.
Time: Salvador took me 3 days (due to an unexpected drive), was planning 4. Primitivo took me 11 days. Muxia + Finisterre took 3 days. I took a day each to sightsee in Oviedo, Finisterre and Santiago. I was not in a rush at all but I tend to walk fast and for long stretches at a time without stopping.
Company: I was alone on the Salvador. There was one other person who walked on the Primitivo at the same time I did. Being nearly untravelled during winter/early Spring, locals seem more friendly/hospitable. Besides the free rides, I also got free food from a restaurant owner, free advice from a few people and lots of hellos and pleasant greetings from all kinds of people. I actually found that as soon the Primitivo ended (at the Frances) locals were less friendly, as traffic increased substantially from all the other pilgrims.
Experience: Absolutely fantastic! I chose the Salvador + Primitivo because they were less travelled and less seen. The scenery was gorgeous, whether it was snow covered or warm and sunny. The more dangerous sections were...interesting but navigable with some thought and care, but those sections were very few and far in between. Overall, an experience I will not forget.
Buen Camino,
Fidel
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