As far back as is relevant.
You're obsessed with unimportant timing. That's so typical of cocaine fiends. ;)
I'd say the press didn't make it much of an issue because they thought it was boring because Obama had already confessed to it, and every candidate, left and right, has admitted to illegal drug use, except the candidate who by far had the most problems with it. The latter could explain why the press didn't bring up drug use -- why should they apply a stricter standard to Obama when they virtually ignored GW Bush's far worse drug and problem and its continued affects on him while he was in the White House?
If Obama's drug use was a worthy issue, why didn't even the wacko right-wing press try much harder to bring it out? Neither Bill O'Reilly, Michael Savage Wiener, G. Gordon Liddy, Ann Coulter, or Rush Limbaugh said nearly as much about it as about Obama's citizenship status. Explain that. At least explain why the ones who without their own drug problems didn't make it much of an issue.
It's funny when people call an opinion or complete falsehood an "obivious fact", as you just did, but to address your contention I have to ask: what left-wing media? The Washington Post has right wingers Charles Krauthammer and George Will, and the NY Times has David Brooks and the bizarre and incompetent William Kristol, the latter hired well after Republican power had peaked. Talk radio has been about 90% right-wing since the 1980s, and think tanks are mostly right wing PR machines. Left-wing Air America probably has a smaller audience than just a few of Rush Limbaugh California outlets alone, and MSNBC was way outnumbered by Fox News and CNN (with kooky and not- so-bright Glenn Beck). Coverage of Obama and McCain varied during the campaign, and while Obama got much more favorable coverage later on, McCain had been doing better earlier. Sarah Palin was initially actually fawned over by the press (glamorous, charming, can-do frontier woman) and wouldn't have been treated as an idiot if she had never spoken as an idiot. Favorability of coverage for Obama and McCain probably tracked opinion polls more than anything else.
IOW your theme about a coup by the press is plain silly.