“Duration, in seconds, that a page resides in the buffer pool”
SELECT object_name, counter_name, cntr_value
WHERE [object_name] LIKE '%Buffer Manager%'
AND [counter_name] = 'Page life expectancy'
The normal values are above 300 seconds (5 minutes) and the trend line should be stable. If the value is below 300, it’s a clear indication that something is wrong
Also, a value drop of more than 50% is a sign for deeper investigation.