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, put more because of restrictions within the popular JAWS screen reader. Many versions of JAWS will break up text into blocks of 125 characters.