I think around 6 months or so. I started a little later with DD1 (10 months, maybe?). I just signed a couple words as I said them, and tried to be consistent with it. I didn't use dvds, though I'm sure those are helpful!
September 2011 Space
I started at about 4 months with my older son (15 months).... he's FINALLY just starting to do them. I do a LOT of signs with him, but I started out with fewer. I do animals, foods, bathroom things, etc.
Just in the last couple of weeks he's finally started doing more, all done, and tonight he signed milk to me! I was very excited.
A girl I know who teaches sign language class (and has education in some sort of children's education) starts at birth. Her son signed back "change" during a diaper change at 4 months.
We did about 5-8 signs with DD1 starting at 7 months. There was no response from her and one day when she was 10 months she just started doing all of the signs. By 15 months she could say those words and only used the signs if she thought we weren't listening.
I liked it because she could say if she wanted food, drink, milk rather than getting upset and making us guess.
DD1 July 2008 (41w3d)
November 2010 (13 weeks)
DD2 August 2011 (33w5d)
We didn't intend to teach Hailey sign language, but she picked up the sign for "all done" case DH used to do it at the end of meals when he would tell her she was done. Then we decided to teach her "more" and "hungry", which was very helpful.
The funniest thing happened though,the sign for hungry was her pointing to her mouth (ok, I don't know if thats the actual sign, but thats what we taught her). Shortly after that she learned where her nose was, and then started tapping her nose whenever she was hungry. It was so cute!
I expect we will teach the same three signs to audrey, but we will have to modify a little seeing as she only has the one hand, and both "more" and "all done" are two handed signs.
I taught DS and DD1 some basic signs like more, please, thankyou, all done, milk, daddy, mommy... just a few basic ones. Will probably do the same with Miss S starting around 6 or 7 months. Both of my other kids were early talkers so once they started talking, we dropped signing. It was really cute to see my DS signing to DD1 asking her if she wanted more or was all done at meal time.
We did a few signs with Catie, I think we started around six months or so. The funny part was, she didn't use them much until she was starting to say the words. Then she would say it and sign it. So definitely not as helpful as I had hoped, but I still think it's a great thing to do, and I plan to do it again with Ronan. When I was teaching before DD was born (I taught a toddler class in a mother's day out program) several of the kids in my classes used signs and it was really helpful.