Please check out the bottom of Jonathan Kehayias's blog page below:

http://www.sqlskills.com/blogs/jonathan/finding-what-queries-in-the-plan-cache-use-a-specific-index/

To know whether your value is good or bad, you have to know what "normal" is. That means that you need to have a baseline value (or steady state as Jonathan refers to it) for PLE and then monitor for situations where it dips below that baseline value.