Is there a set length / restriction on ALT tag content?

[SamPenno]

Something in which is an essential part of any site but I cant for the life of me remember if there is a set / recommended limit to the size if should be or is restricted to.

[littlesthobo]

In all honesty I've never done any serious testing but I've always worked by a rule of thumb of 5 to 15 words max, with the keyword as close to the front as I can get it..