I'd say how much money do you want to devote to pulling your CR's? It varies from person to person.
I think people should pull their 3 cr's at least twice a year. I pull mine depending on if I'm seeking new credit, like a cc.
I subscribe to a credit monitoring service which allows me to pull daily cr's. While it comes with scores, they are FAKO scores which are useless, other than judging monthly changes in scoring.
I like to pull my FICO reports at least once per calendar quarter....just for the self satisfaction of seeing my FICO scores change. For scoring purposes.
For reviewing what is actually contained in the body of the CR itself, while totally disregarding scores, annualcreditreport.com is a must. It gives you EVERYTHING on your CR, including soft inquiries.
(just my opinion)