Hi, Yng,
You have chosen a beautiful Camino to walk. I'm sure you are going to enjoy it very much.
You've already gotten some good advice, and I'll give a few more opinions to confuse things a bit.
1. I think the "average" walker would take anywhere from 26-30 days to walk the Norte from Bilbao (staying on the Norte that is) to Santiago.
2. I've walked both the Norte and the Primitivo. You have to make the choice near Villaviciosa, which is where one option turns south to take you to Oviedo and then on to the Primitivo, while the other option stays on the Coast till Ribadeo. Having walked both ways, I would say there are tremendously wonderful things about both, you just have to make a decision (or wait till you're actually on the Camino and see what the people you've become friends with are going to do). I've got pictures posted from both of them, so you could take a look and see what the differences are in terms of scenery at least.
Primitivo:
http://picasaweb.google.com/laurie.reyn ... rimitivo1#
Norte:
http://picasaweb.google.com/laurie.reyn ... ELNORTE07#
3. Terry's right, unfortunately, there's just no way to predict the rain. I walked Irun to Santiago starting in the beginning of May and had two or three days of real rain, nothing more. That was in 2007. Others walking a few days ahead of or behind me had lots of hard pelting rain, so your luck will depend not only on the year, but where you happen to be when the rain comes in. We often marvelled at our wonderful luck in getting so little rain on the Norte. I walked the Primitivo inSept/Oct and again lucked out with great weather. Cold sometimes, but dry.
6. My biggest surprise on the Norte was the huge amount of walking on pavement. I don't know if it's still the same, but in 2007, there were many days when I didn't get off road. The primitivo is almost all off road.
Good luck and buen camino, Laurie