The Page Life Expectancy alert is triggered when the number of seconds a page stays in the buffer pool without references is below the warning level. A decrease in page life expectancy can indicate an increase in the physical I/O requirements for a user database. The rate decrease could likely indicate that the memory taken away from the buffer pool is forcing database pages to exit the buffer pool prematurely.
Diagnose problems with your SQL Server instance
Microsoft recommends a minimum target for page life expectancy of 300 seconds. This means that any given page in memory is kept in the buffer for five minutes before the buffer flushes the page to disk. A page life expectancy value of less than 300 seconds is indicative of either a memory problem or inefficient query plans.
To enable alerting when this metric is outside its established baseline, select the Baseline Thresholds Enabled (as percentage of baseline) check box in the Alert Configuration window.