You can setup a reviewer for individual pieces of knowledge (or you can do this in bulk). Your identified reviewer will get an email notifying them that content needs to be updated (on the date you select).
In the Knowledge Base record, the 3rd tab deals with reviewers. You can setup your Reviewer, a Review Date, and Repeats Every (the frequency [in months] which the reviewer is notified).
You can also do this in bulk from the main Knowledge Base listing page. Just select the records you want to set a reviewer for. Then you will see bulk actions available.
Once you select MANAGE REVIEWERS from the Bulk Action Dropdown, you will be presented with these options for choosing the desired Reviewer, Review Date and how often it Repeats.
It is important to understand that if you want to trigger the Review as soon as possible, you will flag for Review. The soonest way to trigger the Review cycle without flagging for review is to set the Review Date for a date that is a day after today.