Legally, the government has to reply within 18 months after your application, according to the page about Naturalisation on service-public.fr, an official website (idk if I may post links)

In some cities like Lyon, you can indeed wait 1+ year for the process.

Informally, the Administration / "Préfectures" are infamous for being slow, and requests are processed in each administrative division, so you'd rather not be in a city where there are a lot of applicants.