What is Page Life Expectancy precisely?
PLE is a measure of how long (in seconds), pages are staying in memory.  It is more a measure of the age of the current pages, than a prediction.  But if the current load stays the same you should expect the PLE to stay the same.  

In reality, it changes, the value grows every second, until there is a big flush then it shrinks.

Related:


Page Life Expectancy (PLE), where to start?
SQL Prove memory pressure - high Buffer Cache Hit Ratio, but low Page Life Expectancy
How can you have a Buffer cache hit ratio of 100% with PLE of 103?