Is there a recommended character limit for alt text tags?

[KimCalvert]

I've searched around for a maximum number of characters to use when writing alt tags, but not seeing anything more than "don't keyword stuff." Is there a recommended number of characters?

[Carlos-R]

This is the only case study I have seen about alt tags http://www.hobo-web.co.uk/how-many-words-in-alt-text-for-google-yahoo-bing/

16 words of 7 + 8 characters each