According to this wiki entry, the PASIV device has a battery life of 200 hours. Multiplied by the time stretching factor of 12 given here, that equates 100 days, or using the factor of 20 for strong sedation, about half a year. No matter how many layers deep you go, when the battery in the previous layer runs out you should be out of dream-time, so how can Cobb claim they'll have up to 10 years in the third layer when the 2nd layer battery wouldn't allow for any more than half a year?
Why don't the batteries run out?
Bonus question: Assuming there's an external power supply or the like, how could Cobb and Mal enter Limbo? (related to this question)