I've encountered the Rat-P Green Jackets the most late at night. Next is mid-day. Least is during rush hour (except buses where they seem to be more prevalant during rush hour). Airport trains are also popular targets.

I strongly recommend getting a photo as soon as possible and only traveling when you have the proper ticket - even on buses. The frequency of checks in Paris pales in comparison to Germany (e.g. Berlin, with their open gate systems), but they still make their rounds.