I'll opt for 16 days AND for early summer ;-) Even though my big France trip was mainly very late in the year - October - and I had still wonderful weather most of the time, there are several areas in France (among the famous ones) who seem to be really deserted between end of September and May. E.g. in the Bretagne I saw most of the restaurants with signs like Have a good time til Whitsunday. So there's not much life in those areas. On the other side May and June are stil not the main holiday times in France (July and August) where the whole country goes for the sea and the routes are more crowded.

Baerbel.