Hi chirimiri,
I did the Via Jacobi in summer 2012, but I came in from Austria and joined it at Wattwill.
The track starting at Konstanz can be done in 19 days, according to the German guide I used (which I also can highly recommend:
http://fave.co/17QP7X1), with stages between 20km and 30km.
This guide mainly covers the north route, but the south split route splits from the north way at Brunnen (as you have to take a boat here
) and runs above the "Vierwaldstättersee", via Luzern and past Bern, while the north route covers Brienz, Interlaken and Thun.
They meet again at Rüeggisberg, where the continue together till Geneva.
Personally, I'd recommend the northern route, as you pass the beautiful double lakes Brienzer See & Thunersee, while the south route run to the both large cities mentioned.
But this is a personal choice
And if you're curious, you can also read a bit about it in my blog:
http://wayofmichaelv.blogspot.com
I cover most of this area from day 10-24, with GPS track available
If you need any more info, do not hesitate to let me know!