I purchased Macbook Air 13 in late 2012. It is advertised as having 7 hour battery life. After a few months of use, with maximum charge it gives me only 2 hours.
Is this normal?
If you're not using a lot of power, this isn't normal. On http://www.apple.com/macbookair/features.html#battery it says up to 7 hours, clearly implying that you could get less. The footnote as text as follows:
The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 50%. The standby test measures battery life by allowing a system, connected to a wireless network, to enter standby mode with Safari and Mail applications launched and all system settings left at default. Battery life varies by use and configuration.
So, if you had all of the the brightness on max, CPU topped out, GPU working and storage downloading, I would expect the battery to last between 2 and 3 hours, so you may have a perfectly normal battery or there may be an issue. This sort of thing is exactly what the Genius bar and AppleCare exist to answer on a case by case basis if the online tips don't work or you want to seek a professional opinion and possible repair.
I would download and install Battery Health from the Mac App Store. Allow it to run and determine if your battery's current maximum charge threshold.
In the case that your battery is no longer charging to its maximum amount, I would make an appointment with your local Genius Bar for some in-warranty love.
If your battery is "just fine", you'll want to consider lowering your screen's brightness, reducing the number of open apps and potentially uninstalling Flash, turning off Bluetooth if you don't use it, and making sure your GPU and CPU power settings are enabled.
"Extend the battery life of your MacBook, no matter how old it is" written on June 22nd 2013 by G. Goetz for Gigaom.com, gives a useful insight into what can be done to preserve or possibly extend the battery life of your Macbook Air including:
Additionally "What happens to your Macbook Air battery in a year" written on 15th August 2013 by L. Hockenson may also provide useful insight.