Import RSS Feed Programatically
There is an option in feed import module settings (admin/config/services/feed_import/settings), where you can set to import your RSS when the cron runs ("Cron import" checkbox). 

If you do not have access to crontab, you cannot schedule cron from server side, but if you set cron interval at admin/config/system/cron, cron will be also triggered on, when the time elapses and there will be site visitors on your page (cron will be triggered by one of the site visitors).

Source here.

Edit:
If you would like to run cron each 5 minutes (or other custom interval), you will need custom module and implement hook_init(), which is executed on every page load, when the module is active. See the example:

function mymodule_init(){
  //get variable content (last cron run time), if not exists, return 0
  $last_cron_run = variable_get('mymodule_cron_run', 0);

  //if it was more than 5min before
  $five_minutes = 5 * 60;
  if($last_cron_run + $five_minutes &lt; time()){
    //run cron
    $result = drupal_cron_run();

    //do with $result what needed

    //set last cron run time to variable
    variable_set('mymodule_cron_run', time());
  }
}