I've separated this into four different types of people.
1. The Student
2. The Explorer
3. The Muddler
4. The Snub
The Student
This type of person, I am the most envious. You can always find an English/French dictionary within arms reach of them. They have probably spent months intensely studying various French courses before moving to France. Their French dialect is textbook style, and the French bureaucratic love of paperwork offers no fear.
The Explorer
Like Sir Walter Raleigh, the explorer places themselves in foreign environments and finds strategies in which to understand their surroundings. They learn by experience and probably have no idea what a French word looks like in written format, but are already speaking with ease and even picking up the local dialect. They learn the same way that a child learns to speak, out of necessity. Being part of the French workforce, watching French TV or joining one of the many French social clubs allows you to be constantly hearing spoken French and you learn without even knowing that you are. The Explorer is a creature with a social habit.
The Muddler
The Muddler says 'Life is too short to stress'. While the student type and the explorer type are conversing freely, the Muddler is still using elaborate hand gestures to get their point across. If you do not need to work in France, and you have a partner or friend in which you can enjoy intelligent conversation in English, then this approach can be the most relaxing and also the funniest. This is the category that I myself fall into. I have French friends and enjoy conversation, and our biggest laughs are often at how bad my French is. Just the other day I put my hands over my ears and wanted to say that I am not listening, but it came out in French 'I have closed my eyes'.
The Snub
While the Snub-type love the French lifestyle, they degrade the value of the people to such an extent that they don't even bother to muddle through with some basic words. I once met a man who has lived in France for six years, and the only word that he has bothered to remember in French is 'Bonjour'. There are enough English-speaking 'cliques' in each village for this type of person to live quite happily in ignorance of the French language.
In conclusion, the question is "How much French do I need to know to live in France?" The answer is "None, but any words that you pick up by muddling through, exploring or studying will only serve to make you life in France that much more enjoyable."
Richard Hull is the author of Your Property in France