Background Task 的运行时间在iOS 6及以前有大约10分钟左右，在iOS 7中有180秒。

Reference:

1. Multitasking in iOS 7

http://www.slideshare.net/mremond/multitasking-in-ios-7

2. Background task interval period in iOS 7

http://stackoverflow.com/questions/18815384/background-task-interval-period-in-ios-7