Each application has its own cookie store. So if there is no expiration on the cookie, and you don't delete it, and the app isn't deleted, then there's no problem with using it for as long as you like.
EDIT Here are some links for more opinions and insight:
My opinion on the matter is that the mobile environment is fundamentally different from the desktop environment. "Quitting" a mobile application is in no way similar to quitting a desktop application. Quitting a mobile application is similar to switching focus on a desktop. You would not expect to re-authenticate every time you pressed Cmd-Tab.
Limiting the life of session tokens is a valuable security precaution, but is correctly implemented on the server side, not the client side. If the server is designed to allow a session to persist indefinitely (because the desktop app is never quit), then there is no reason to not continue the session on a mobile platform in a similar way.
Note that there are other solutions, such as storing the user credentials in keychain so that you can reuse them. This is appropriate in many cases, but it actually is a lower-security solution than persisting the session token indefinitely. If you're going to hold onto an authentication credential forever, it's better that it be a single-purpose token (i.e. a session cookie) rather than a multi-use username and password.