Found in EmailLabs' excellent newsletter Intevation Report:
Question: I understand about using alt text to get my message across if a reader blocks images from downloading, but is there a limit on the number of words I should put in the text?
Answer: For most images, you should need only a few words to describe the image ... four or five, likely no more than 10. Try to describe both the image and what you want readers to do.
Remember: Images should always support the message copy and never replace it. So, your reader who doesn't enable images should still get the point of your message, even without the images or alt text, from the message text.
As far as how many words to use, use this guide: The smaller the image, the fewer the words. And remember that some email clients, including Hotmail and Outlook, add text to the front of the alt text which can reduce the impact. Always test your message with images off before you hit "send" to make sure you're getting your message across.