Traditionally, stores like supermarket are open from 9AM to 12:30PM, then 2PM to 7:30PM. If you are lucky, they might be open on Sunday morning, but you better check...
If you are in a big city, you might have more luck checking out "l'arabe du coin", which literally means the "neighborhood's Arab"... Let me explain: in Paris, you'll find little "épiceries" (grocery stores), usually open all day from 9AM to 8PM, also sometimes Sunday, and these stores are typically run by Muslims, hence the "arabe du coin"... Not too PC, but you're in France, right?
Note that "les boutiques" are also closed at lunch time - not in big cities though - and are very often closed on Monday (they did work on Saturday, so you know, they can't forfeit their 2 day weekend...)
Lastly, everything will be closed on Holidays, which are plentiful in France. Everything... except maybe l'arabe du coin...
