Hi Leo,
I have done the Canal de Nivernais about 6 years ago over a 2 week period.
You will need 2 weeks to do a complete round-trip. ITs fantastic with great little towns/villages, friendly locals and good amount of locks (not too many). Its very easy. Suggest you rent bikes as it can be a reasonable distance to get fresh bread in the morning or visit some towns. One of the best holidays we had done. PS, make sure you get a good sized boat, its normally pretty cramped as is, it pays to pay a few extra bucks to get the next size up from what is recommended.
Good luck.
Peter