With Mountain Lion's imminent release any day now, according to a close friend, I'm happy to report there is evidence Mountain Lion has been optimized for improved battery life -- particularly noticeable on MacBook Air machines. Previous to Mountain Lion, MacBook Air 11" noticeably drained battery quite quickly. With Mountain Lion, it seems the battery life is much more respectable. If I had to guess, I would go on a limb and say up to 25% more.
Charging would be necessary by 12noon under normal usage and it seems it lasts through early afternoon. We'll have to see in the next few days if indeed this is a common finding upon early adopters of Mountain Lion.
UPDATE 8/7/2012 In lieu of people having battery problem with Mountain Lion:
I didn't realize this, however as I initially started using OSX Mountain Lion I do recall making a change because my MacBook Air battery would train really fast while in sleep mode closed. Meaning, I would close it at night with say 90% only to wake up in the morning with 75%. It seems by default the memory remains powered unnecessarily after going to sleep. I'm not sure if this is a necessary setting for HardDisk based MacBooks, but for MacBook Air's this is certainly not the case as the computer should save an image of memory to the SSD quickly (e.g. hibernation) and then power down the system components.
I changed my power management setting manually to ensure it hibernates while powering down and ever since I have no longer had battery issues with my MacBook Air.
First, check your current power settings by running this command:
pmset -g custom
And you should see something like this:
There two sections "Battery Power" & "AC Power" along with each of the various settings for each section. If you notice, hibernatemode for the battery section is currently 25. This is the setting you want to have for your MacBook Air. Ever since setting this to 25, I've woken up with 100% remaining after leaving the MacBook Air to sleep for many hours.
If you look into the details of pmset command, you will find the three recommended settings as shown:
Clearly, us MacBook people, particularly the MacBook Air people or those with SSD's as storage, would want to have setting 25 and it seems it is only configurable manually via pmset. Here is the magic command. Run this with the command line and you should no longer have problems with battery.
sudo pmset -b standbydelay 900 standby 1 hibernatemode 25
The stand by delay of 900 seconds simply means it keeps the memory powered for 900 seconds (15 minutes) in case you return to your MacBook quickly. This will give the instant on impression. After 900 seconds, the memory is powered down and the machine must restore the memory from hibernation which simply means it takes an extra second or two to power up. If you like this convenience, you may want to increase/decrease the 900 value by however many seconds makes sense for you.