Here is the "official" page about renewing your carte de séjour:
Renouvellement de la carte de séjour temporaire ou du droit au séjour du titulaire d'un visa valant titre de séjour - Service-public.fr

They say you need to apply for renewal during the last two months before the expiration date. So a month and a half before sounds about right to me! I know people who have literally waited until the very last minute to renew (and gotten a certain amount of grief, but they still got renewed), and I know folks who have tried to renew earlier than that two months and were told they would not be allowed to do so. It all kind of depends on your préfecture.
Cheers,
Bev