Alt tags to give a description of an image so that if a user has images turned off or has a visual defect then they can still find out what the image is of.
Alt tag is use for promoting image hope you know google can't read your image that's why some peoples are using content in alt tag and title tag it's a 1st reason and another is it's use for our visitors we can add in alt what we want to tell our visitors and for visitors convenience. it help to make your site visitors friendly and I don't think so here is any limit of words in alt tag but make your don't add so much keywords or text which can look like odd remember one thing in your mind you are creating your sites for your visitors not for only search engine.
From what I understand there is no limitation on characters used. But your keywords shouldn't be that long even if its long tail. And it'd be wise not to spam all the pictures on your webpage with the same keyword for alt text. Renaming your image as the keyword helps as well. Alvin Huang
They're NOT TAGS - they're attributes. They're only supposed to be used to provide a textual equivilent of the image in question for those who cannot see the image. This includes the blind, people who have images disabled or are using a browser that does not support images (NCSA Mosaic, Lynx, etc.), and search engines, just to name a few. The general rule of thumb though is to keep them as short, sweet, and to the point as possible.