Or you could wash your hands after eating.
Forever
Or until battery dies
I always make sure to suck the Cheetos dust off my fingers really good (like, using my bottom teeth to scrape the dust off as I suck) before playing, so yeah, my controllers typically last a long time.
Edit: Oh, and yeah, be sure to wipe your fingers on your shirt after getting the Cheetos off. Obvious, but I wanted to make sure people knew.
About a year or so. I'm very OCD with my gadgets, try to keep them clean and in great condition.
My NES controllers and Zapper still work. All my other controllers work, too.
The Wii U controllers will probably fail first, due to the internal batteries.
PS4 controllers - 40 hours max, break R2 button, return for new one, rinse repeat.