Therefore, it is important to be succinct and descriptive. In considering Search Engine Optimisation do not try to keyword stuff your Alt Text. The search engines understand semantics, therefore they will value similar words to your main keyword that you are optimising for. Be as natural as possible and do not repeat your Alt Text for different images.
While there is no limit to the length of your Alt Text, it is recommended that you aim for 125 characters or fewer. This is less because of the search engines, but more because of restrictions within the popular JAWS screen reader. Many versions of JAWS will break up text into blocks of 125 characters.
Be as descriptive as possible about your image or file. This will help the users understand what is being displayed, though it will also help the search engines determine what is on the image, but also to determine the topic of surrounding text. Search Engines like Google, therefore, place a high value on Alt Tags and this ultimately will assist in the search engines indexing your page.