How long can an “alt” attribute be?

The HTML (Hypertext Markup Language)specification does not define a maximum length for “alt” attributes. Current versions of the leading screen reader programs have no limits on the amount of alternate text they will read. However, Freedom Scientific JAWS divides the “alt” attribute into distinct chunks of 125 characters each (excluding spaces), and reads them each separately as if they were separate graphics. This behavior, though it ultimately provides access to the entire “alt” attribute, can be confusing for the user. Also, keep in mind that “alt” attributes additionally benefit sighted users who have images disabled in their browser settings, perhaps in order to minimize download time. For these users, if the height and width of an image is specified, both Internet Explorer and Mozilla truncate the alternate text to fit within the size of the image placeholder. If no image width is specified, Internet Explorer displays the entire “alt” attribute on one line, which may result in a web page that is extraordinarily wide and disorienting.

All of this provides technical justification for keeping the length of “alt” attributes to a minimum, at least shorter than 125 characters in order to avoid JAWS subdividing it. This simply reinforces a common request from screen reader users: Keep alternate text short and sweet. Alternate text should provide equivalent access to the content of an image, but must do so efficiently in order to avoid burdening screen reader users with extraneous information. For images that require more lengthy descriptions, the appropriate HTML attribute is not “alt” but “longdesc” (short for “long description”), which allows authors to provide a long description of an image on a separate page. This is particularly useful for complex images such as graphs and charts. Screen readers typically announce the presence of a long description when available, and provide users with the option of reading it.

How Many Words In ALT Text For Google, Yahoo + Bing?

Google seemed to count the first 16 words in the ALT tag (well.. the Alt tag is an HTML Attribute) – of the image on this page – in this instance and interestingly in the snippet Google uses it does seem to completely cut of the rest of the ALT

EDIT – that’s 16 words of 7 + 8 characters each. which might prove useful if you are using ALT tags to describe complex images.
That’s potentially plenty of space to describe images properly for accessibility purposes and seo effectiveness.