they've been going strong since 2009, so it's been 6-7 years. with their recent successful book & tour, it's unlikely that they'll "burn out" anytime soon. I'd give them another good 5-10 years before they retire from youtube
you know that there has not been a fad that has lasted an average human lifespan since like 1500 right. and that was only because people didnt know how to progress back then
Sorry.