How long does it take for Riot to make a new champion skin?

Hello, I am a student currently doing an extended project qualification (EPQ) on microtransactions and how they have affected the gaming market. I am just asking this so I can have a better understanding on the amount of research and development that goes into available DLC, that way i can relate this part of my essay to sunk costs as my project is about economics. Any information would be appreciated as long as it is reliable, thanks.