I did some with DS starting when he was a few months old. He could sign milk by the time he was 7 months (which was much nicer than his previous method of jamming his head into my chest or pulling on my shirt) and picked up some more around a year (more, done, please, book, and sleepy). I didn't really teach him any other ones. I will likely do the same thing this time around - teach some of the more "essential" signs, but nothing more. DS took a looong time to actually start saying the words that he knew how to sign, even though he could say lots of other words (he didn't start saying "milk" until two or three weeks ago, for example, even though he's 2 and talks all day long - he would only sign it until then).