How to write a drupal cron job to fetch data from a link
Create a simple Drupal 7 module, there are a lot of examples for this (i.e. here). Then hook into cron with a function mymodulename_cron() and inside the mymodule_cron function fetch your json data.

Your mymodulename_cron() will be executed each time, the drupal cron is run. This can be done from outside or from the admin/reports page. Look at /admin/reports/status for info about this.