Hi,
Unfortunately, I couldn't find any official documentation regarding cache policy when using Visual Search. However, in Microsoft's Privacy Statement under Other important privacy information, it mentions the following:
“Microsoft retains personal data for as long as necessary to provide the products and fulfill the transactions you have requested, or for other legitimate purposes such as complying with our legal obligations, resolving disputes, and enforcing our agreements.”
I believe this retention policy is what Microsoft applies across all services, including Bing Visual Search uploads. Other than that, there's no specific or detailed information available about how long image data or cached content from Visual Search is stored before being deleted. I’d recommend checking the Microsoft Privacy Statement as well, since you may discover further information there that addresses your concern.