I want to persist access token life time remain same once i got after login.is there any way to do it ?. other thing is can i get access token from back end without opening RingCentral Popup or login page ?
"persist access token life time" is impossible for security reasons. What you can do is to refresh your access token periodically to make it always non-expired. Refresh token can live as long as a week, so you can refresh access token once a week to make it non-expired.
Also, if the access token and refresh tokens are captured and maintained at the backend (maybe in a db), you can run a script to keep the access token/refresh token pair active and the user's auth will not expire.