Civirules: how to move contact into NEW group XX days after added to group
A rule can't be created without indicating an explicit trigger - there is a cron job for CiviRules whose description reads: trigger civirules cron triggers.

A few things about your condition: 


If you are not 100% sure that the created date will always equal the activity date, you're probably better off using the activity date instead since this is user-driven whereas created date is system-driven
Using an operator of "Greater than or equal to" will allow you to capture any retroactively created/updated activities (another argument for using the activity date instead of the created date)
"Compare Value" assumes system date, so it actually needs to be negative to compare the previous 90 days (when you save the condition, you will see the date 90 days ago listed as follows: Activity.activity_date_time >= YYYY-MM-DD). 


If you find that the activity based rule is not working out for you, you may wish to consider the development of custom triggers, conditions and actions as documented HERE.

Hope this helps,
Tamar