You often see Page Life Expectancy referred to as an interesting performance monitor counter. And it can be! It indicates for how long a page is expected to stay in cache, from the time it was brought into cache.
But just looking at a snapshot value doesn't say that much. It might be high, but that is because you haven't had a high turnover of you pages for the past couple of hours. Or the other way around, you happen to look just after a very rare monster query. Furthermore, having a log can show you how PLE changes over time.
"The app was slow at two o'clock today."
You look at the PLE log and you see how PLE dropped at that time, which indicates that something was executed that did lots of I/O. Or you don't see PLE drop at all, and can assume that it probably was a blocking situation instead.
Some of you already have performance monitoring solutions that include PLE logging over time. For you that don't, I created a very simple solution. It involved a table, a scheduled Agent job and queries to read the data. That's it.
You find it at http://www.karaszi.com/SQLServer/util_log_ple.asp .
Improvements and comments are of course appreciated. :-)