Last half of 2015 probably, or first half of 2016, depends on the 3DS' decline and the PS4's peak sales levels. Even the PS3 managed to sell about 14 million in its best year, something the 3DS is having huge problems beating despite having massive amounts of software, historical price cut and several hardware revisions, not to mention no competition to speak of, I don't understand why anyone would see this as impossible by any measure; the 3DS is likely headed down and the PS4 will probably peak in 2015 or 2016, there will be a crossing point where the PS4 should manage to beat the 3DS consistently through the year.
For anyone saying "never"; the 3DS is not the DS and it's not doing as amazingly as you think, the DS had three consecutive years of near 30 million sales, the 3DS is struggling to reach half that. On the same note; the PS4 is not the PS2 either, it will never sell 20 million in one year, I expect it to have a sales pattern similar to the PS3 but with a flatter sales curve and with higher sales overall but a shorter life cycle.