] The rovers' lifetimes will depend on factors such as
] durability of their electric motors, cumulative effects
] of severe day-night temperature swings, and the amount of
] available power from solar cells, Naderi said. For an
] added $15 million, the $820-million mission has been
] extended to Sept. 13. Mars will pass behind the sun then,
] Naderi said, cutting communications.

Someone asked me about this a few weeks ago. They are apparently running way longer then expected, and NASA has funded further work with them.