There is no definative answer when it comes to the amount of time you will need to complete the journey from SJPP to Finisterre. Falcon's response demonstrates a range of opinion, but it really depends on the individual. Those who walk the Camino, and for that matter post to this forum, vary greatly in terms of age, physical ability, the desire to slog casually along vs. keeping a brisk pace and other factors.
Suggestions in guidebooks try to apply collectively to the greatest number of people; taken on an individual level however they may suggest too much or too little territory to be covered in the average day.
I've done the same walk in March 2010. Issues worth noting that may affect your timing:
1. There were a few days of heavy rain coupled with gale force wind, periods of heavy snow over mountains as well as many days of glorious sunshine. By far the biggest obsticle was mud; not just on the surface but in some areas with depths well above ankle level. This both because of the heavy rains as well as snow melt streaming down from higher grounds. You may decide you want to take a day or so off along the way, particularly if you encounter severe weather or if you just want a break from it all.
2. Life seems to go into hybernation during the winter. In many places you will find that it has not yet sprung alive. At the end of the day's walk there may be little to do. It is unlike other times of year when outdoor cafes and other activites make it conducive toward stopping earlier to enjoy the local offerings. Instead, feet and energy level willing, you may want to walk longer days than you otherwise might at a different time of year.
I've repeated the same Camino in autum of 2010. Completely different demographics of pilgrim. In March they were on average much younger, appeared heartier, and in general maintained a quicker pace (albiet there were still those of all ages and all paces --- this is my general observation).
3. Although the way may be isloated, companionship is always available. You'll have the option of just how much time you want to spend in self introspection. You may also want to stay within a certain group of new found friends and so keep pace with them.
Personally I enjoyed the energy that came with maintaining a brisk pace. I was there to walk and wanted to keep doing so to the fullest every day. On average I put in eight to nine hour days, allowing for coffee and lunch stops. I found equal minded people and had no lack of companionship whenever desired. The trek from SJPP to Santiago took 21 days, then a further three days to Finisterre. Many might argue that this sort of pace rushs things too much. It just worked for me but may not for the majority of people. I was fortunate not to have any significant foot or other health concerns. On the flip side there are those that would prefer twice that amount of time.
Not to confuse an answer to your question, but again there is such a wide range depending on the individual. If you can spare the time and have the financial means, overplan the amount of days that you think you might need. It is nice to spend two or so days in Santiago either before or after going on to Finisterre. And if time permits there are other places within reasonable travel distance from Santiago in which to spend a few of the excess days. Having extra time myself I took a train from Santiago to Porto, Portugal and spent a few days there, then went onto Fatima for a day. In the later period of the month the cities and towns wake out of their winter slumber.
Buen Camino!
