Is there a recommended description length for alt text?

The Web Content Accessibility Guidelines (WCAG) advise that alt text should be brief. For non complex images where a simple description suffices, you want to keep alt text to one to two sentences, if possible. Screen readers read content as a single string, so there's no way to pause the text or go word by word. This results in alt text presented as a chunk of text from beginning to end. If the description is long, it would create a less-than-ideal user experience. 

When dealing with complex images where users would benefit from a significant amount of detail, you should provide the information as visible text on the page. The image can then be coded as decorative or include brief alt text directing users to more information. For example, “Horse jigsaw puzzle, full description in product details.”