Based on the information provided, there is no specific recommendation for how often liver tests should be performed while taking Lipitor (atorvastatin), a type of statin medication used to lower cholesterol levels. However, it is generally recommended that individuals taking statins have their liver function monitored regularly, particularly when starting or increasing the dose of the medication.

According to the National Health Service (NHS) of the United Kingdom, "your doctor will usually recommend that you have a blood test before you start taking statins to check how well your liver is working" (source: [2]). Meanwhile, a study published in the Journal of Clinical Lipidology notes that "liver enzyme tests are recommended before starting a statin and usually within 12 weeks of starting or increasing the dose" (source: [3], page 3).

Therefore, it is important to have liver function tests performed before starting Lipitor and to discuss with a healthcare provider how often to have these tests performed while taking the medication. It is also important to note that Lipitor and other statins are generally safe and effective medications, and that liver function abnormalities are rare (source: [1]).

Sources:

* <https://www.drugpatentwatch.com/p/tradename/LIPITOR>
* <https://www.nhs.uk/conditions/statins/considerations/>
* <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5325661/>