Any time at all
You can climb all year round in font however the summer can be very hot and the winters very cold. What time of year you go depends as much on what you want out of your trip as it does the weather. If you're wanting to just relax, do circuits and top up your sun tan, summers in Font are beautiful. Temparatues can get into the 30's though so there obviously won't be as much friction as a crisp winter's day but find a shaded problem and get out first thing in the morning and there's no reason for not getting your project done! Plenty of hard bouldering still goes down in the summer.
A trip in the winter could reward you with blue skies and the best conditions the forest has to offer but it's obviously much more likely to rain or snow and if it does it can often take a while for things to dry. The stats say that April is the driest month of the year and May is the wettest but there's not really much in it to be honest.
Most people opt for Spring or Autumn to try and balance the likelyhood of rain and good bouldering conditions. For this reason it can get busy at some areas but venture a bit further than Bas Cuvier and you'll always be able to find a boulder to yourself.