How long should the Image Alt Text be for SEO?

[Ytellsocial]

When it comes to creating the alternate text for images, what is the character length that I need stay under to rank well?

[MichaelC-15022]

There isn't really any limit, like there is for page titles, meta descriptions, etc.  Typically you'll want the ALT text to explain what's in the image--the original purpose was to show the user what the image was before it was downloaded, and also for vision impaired folks, the screen readers would read to them what was in the image by reading the ALT text.

If you're looking for the image to reinforce the relevance of the page for the page's target topic, then make sure that the topic term is in the ALT text, usually as part of a long phrase or sentence.  If you're looking for the image to rank well in Google image search, then I'd keep the ALT text to just what the target term is (and of course make sure the page title reflects that term as well).

[EGOL]

This is just my feel for this after watching my own sites pull in thousands of visits per day from image search, but I have not done systematic study.... and Google could have changed even if I did systematic study.

First, I say that character count is irrelevant.  What you need to focus on is the keywords you want to  be visible for.

If you are optimizing for image search, I would use very specific keywords.  The exact keywords that you want your image to rank for. AND, most important, the exact keywords that people will relate your image to.  If you call it a duck and it ain't a duck, that blows the value of alt text.  Google knows a little about this stuff, even if they make the news by messing up once in a while.

If you are optimzing for websearch, I would use the specific keywords for my webPAGE on the highest image in the code of the page.  It is really important to give people an image that their query relates to.  If your page is about ducks and you image is a weasel, they are going to leave.

After that, it I mix it up, keeping relevant to the image and relevant to the page.

[AlanMosley]

The images on your page should be relevant to your pages content, so describe them with an alt tag naturally will add relevance. The length is not important, but I would not stuff they will a long list of keywords.