Lipitor is a medication used to lower cholesterol levels and reduce the risk of heart attack and stroke. According to DrugPatentWatch [1], Lipitor's patent expired in 2011, and it is now available as a generic medication. Lipitor is commonly used to prevent clotting, which is a common cause of heart attack and stroke.

The prevention effect of Lipitor varies depending on the individual and their medical history. According to UpToDate [2], Lipitor is not typically used as a treatment for acute ischemic stroke but can be used as a preventative measure for future strokes. It may take several weeks or months of taking Lipitor regularly to see a significant reduction in the risk of stroke.

The NHS [3] states that Lipitor is commonly prescribed to people who have had a transient ischemic attack (TIA) or mini-stroke to prevent future TIAs or strokes. Lipitor is usually prescribed alongside lifestyle changes, such as a healthy diet and exercise. The length of time it takes to see the clotting prevention effect of Lipitor can vary depending on the individual's health and lifestyle factors.

In conclusion, the length of time it takes to see the clotting prevention effect of Lipitor varies depending on the individual's medical history and lifestyle factors. It may take several weeks or months of taking Lipitor regularly to see a significant reduction in the risk of stroke. Lipitor is commonly used as a preventative measure for future strokes and is often prescribed alongside lifestyle changes.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.uptodate.com/contents/ischemic-stroke-treatment-beyond-the-basics
[3] https://www.nhs.uk/conditions/transient-ischaemic-attack-tia/treatment/