Joy accounts for two-fifths of overall Mood. A Genopet with maximum Joy will have 40% of their maximum possible Mood without taking other needs into account.
Playing is the player action which can increase Joy. Joy decays every 48 hours, taking 160 hours to fully decay to 0. Similar to Nutrition, when Joy is 0, a Genopet's Mood is effectively capped at 60% of its possible maximum.