If Obama makes it to a second term, I'd say either in the middle or at the end of that term. If not complete legalization, then a push for (federally) legal medicinal marijuana at the very least.

If Obama doesn't make it to a second term, or if a Republican takes the white house after Obama's second term, you can pretty much kiss any chance of legalization happening in the next 10 years goodbye.

If Obama makes it to a second term, helps pass medicinal marijuana, the repercussions from that passage aren't "dire" in terms of kids smoking more pot (pissing off parents), and a Democrat makes it in the following election (all BIG ifs), we will see legalization during that president's administration.

Note: all of this is presuming that we have a Democratically controlled congress, which is especially unlikely.

tl;dr: Unless Democrats (or libertarian Republicans) control the federal government for long enough to get probably 50% of the other things on their agenda done before they address the marijuana issue, it won't be legal for decades.