The Feds won't have to - if each state continues to make it legal/regulated, it's the same effect as descheduling it. It's kind of ingenious, actually, on the part of the Federal government.
It's not going to happen in 2015 though, unless it gets tacked onto some other crazy spending bill, which is unlikely. It'll probably happen sometime between 2016 and 2024.
When the US passed the 55 mph speed limit, it was generally ignored by motorists and wasn't really enforced by a lot of states--and it was eventually repealed. Key difference though is that the feds never had enforcement authority unlike the case with the DEA.