Update CCK Field Every 90 Days
You need to set up cron tab on your server may be once a day
  use hook_cron() to add your custom code write query to grab nids for which field values needs to be updated.
  use node_load_multiple() to load all nodes with the query returned nids
  update the field values and save the node again in a loop.
  set the crontab to run at the time when your site receives least traffic, this way it will not be performance issue as well.