It's essentially a myth [0]. According to the latest stats from the European Commission [1] the French work 40.4 hours/week on average (including paid/unpaid extra hours; excluding meal breaks and travel time), which is close to the EU average of 41.4 hours.

[0] http://www.bbc.com/capital/story/20140312-frances-mythic-35-...

[1] http://ec.europa.eu/eurostat/tgm/table.do?tab=table&init=1&l...