Point in time Expiry!

I created a point in time with expiry set as 15 min, and i have observed that if the query(this query is not extending the pit ,i.e. keep-alive ) using the pit is running close to the expiry, then this pit is not getting expired.

P.S I have the understanding that pit will expiry at the given time, it will not extend the pit if keep-alive is not used.