When home button pressed and app put in multitasking menu (session still active)
-(void)applicationWillResignActive:(UIApplication*)application
After 3 minutes of inactivity in multitasking menu (not being re-opened) you can kill your session here
-(void)applicationDidEnterBackground:(UIApplication*)application
When app is re-opened use this method to compute the difference between going on background and the time your app gets reopened. If this time interval exceeds lets say 3 minutes kill the old session and create new one
-(void)applicationWillEnterForeground:(UIApplication*)application
User deletes the app from multitasking menu kill your session here
-(void)applicationWillTerminate:(UIApplication*)application