Battery life of macbook air with different hardware configuration
There was a pretty good study done on this specific question done here. Also, it's important to note that Apple's stated battery life is based upon very light workload tests.

http://www.anandtech.com/show/7113/2013-macbook-air-core-i5-4250u-vs-core-i7-4650u/4

Key take aways


Idle/light workload battery time is nearly the same
Medium and heavy workloads the time is reduced (which makes sense since the CPU is doing more)


Spoiler images


(source: anandtech.com)

(source: anandtech.com)

(source: anandtech.com)