Hi I am using Connected App for Using Oauth2 connection. where Third Party can access rest API's devloped in salesforce using connected App.
while we are stablish connetion with Third part app using Connected App token is avilabel for 20 minutes only.
my connected app 
Refresh Token Policy: Refresh token is valid until revoked. so why token is expired in 20 minutes.
I want to use same Token until I revoke it. so Third party App dosen required connection every 20 minutes.