Most likely? 2016. Ultimately, Obama will likely win reelection. There's no way Republicans lose 2016, though. The last time a party won three consecutive terms as President was in the 80's with HW, who only got 1 term and faced a weak opponent in Dukakis.
The 1960, 1968, 1976, and 2000 elections were all so close that the slightest change in circumstances might have tipped the election the other way. So there really isn't much evidence to suggest that the challenger party should be considered that much of a favorite when they're running against a party that's been in office for two terms.