title: OAuth REST access_token and instance_url expiry time?
body: <p>I am working with Oauth2.0. In that i could able get the access_token and instance_url. Is there any expiry time for these two. </p>
  
  <p>Can I store them and use it for all my REST calls without getting the new access_token and the instance_url. Is it possible for me to use it for long hours or even days. </p>