It seems new MacBook Air owners, myself included, are experiencing strange behavior with the battery in the new MBA.
While the expected usage of 3 - 5 hours is holding true, the time to charge the battery can be as long a 9 hours in some cases. While the cause of this strangely long charge time is for the moment unknown, it is the apparent randomness in the time taken to charge the battery that seems to be causing the most confusion.
For example, running the MBA down to within a few % of 0 and plugging in the charger one can get an estimated time to charge from anywhere between 3.5 - 9 hours. There seems no apparent cause for these extra long times, but given that the next charge takes only about 1/2 the time of the last, one can only suspect that not all is well in the MBA world.
It has been theorized that the MBA is trickle charging the battery for the full 8-9 hours instead of fast charging the first 80% of the battery and trickle charging the remaining 20%. This later would give the more reasonable time of 4 hours that users sometimes see.
It has yet to be seen, with the MBA being so new, if this problem will settle down, if it is part of the design (Apple has yet to comment, and Applecare is aware of the issue and monitoring) or if the battery (or batch of) has inherent problems.
It is hoped that a battery firmware fix could solve this issue, but if it goes deeper than that, Apple may have created a major headache for its users as the MBA battery is non removable.