Well, it depends. If Sessions goes after pot as hard as he's pontificating about it, I could easily see a huge backlash against them. That, in turn, would elevate legalization to an actual campaign issue, and something that even Democrats wouldn't be dumb enough to ignore going into 2018-20. Assuming all that's done and the Dems win in 2020, they'd likely make a push towards legalization as early as 2021/2022. So, assuming all of that, 5-6 years away.
Assuming that the Sessions talk is just empty threats, probably over a decade out. Maybe 15 years.
The poster formerly known as Kakarot181: July 2, 2002 - March 14, 2012.
Irregardless, for all intensive purposes, I could care less.