Actually, decay rate is based on an individual basis now. Every Pokémon CP has a unique decay rate based on its CP. The lower the CP, the slower the motivation loss. That's why oftentimes, I'll put a wobafett in a gym that doesnt get fed often, because it has AMAZING HP, (more than my 2633 snorlax), but its motivation goes down SUPER slowly, since its max CP is very low <1000. It can actually last for days without losing too much motivation.