Page life expectancy is an instantaneous measure of how long (in seconds) a data file page is expected to remain in memory in the buffer pool (also referred to as the buffer cache).
ie, if PLE is 400, then on average a page is expected to remain in cache for 400 seconds.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability