AF only allows pre-assigned seating on long-haul flights, in all classes of service. Pre-assigned seating for intra-European flights is available only in business class. AF Tempo Challenge is an intra-European full Y fare class that allows for the same food and drink as business class and a separate section of seating between business class and regular coach (also known as Tempo).
For seating in Tempo, OLCI starts 30 hours prior to departure for almost all flights except those itineraries going to the US where it starts at 24 hours prior to departure. The first few rows of coach usually are blocked for FB elites (or so they say), but in my experience, anyone can choose them as the OLCI system rarely really blocks them off properly.