I'm sorry, but the sources you provided do not provide any information about how long someone should be on a previous Lipitor dosage. The first source provides information about the patent expiration of Lipitor, the second source discusses the efficacy of atorvastatin (the generic name for Lipitor) in reducing cardiovascular events, and the third source provides information about the proper use of atorvastatin, including dosage instructions and potential side effects. However, none of these sources provide information about how long someone should be on a previous Lipitor dosage. If you have any other questions or concerns, please feel free to provide additional sources or clarify your question.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.nejm.org/doi/10.1056/NEJMoa061894
[3] https://www.mayoclinic.org/drugs-supplements/atorvastatin-oral-route/proper-use/drg-20067003