How fast is data leving the cache, when it not access again.

With this query, you can see how fast data will get expire, and overwritten by new data in the cache.

SELECT  @@servername AS INSTANCE
,[object_name]
,[counter_name]
, UPTIME_MIN = CASE WHEN[counter_name]= 'Page life expectancy'
          THEN (SELECT DATEDIFF(MI, MAX(login_time),GETDATE())
          FROM   master.sys.sysprocesses
          WHERE  cmd='LAZY WRITER')
      ELSE ''
END
, [cntr_value] AS PLE_SECS
,[cntr_value]/ 60 AS PLE_MINS
,[cntr_value]/ 3600 AS PLE_HOURS
,[cntr_value]/ 86400 AS PLE_DAYS
FROM  sys.dm_os_performance_counters
WHERE   [object_name] LIKE '%Manager%'
          AND[counter_name] = 'Page life expectancy'

Witch the query you can see how long time data will be in the cache/buffer.