Q: My macbook air's battery doesn't last as long as it should

Runaway applications can shorten battery runtime, affect performance, and increase heat and fan activity - Apple Support

AppleTechDenver wrote:

 

I have the MacBook Air 11' and on http://apple.com it says the battery is supposed to last 9 hours.

Did you read carefully?

Note the words 'up to' not 'will' last 9 hours.  Apple makes these measurements based on very minimal resource intensive applications.  My experience that battery run time is much less than these unrealistic advertising claims.  Battery run time will be a function of the number and types of applications that are in use.

Try a SMC reset:

http://support.apple.com/en-us/HT201295

That may improve things somewhat.

Ciao.