If you are just signing as an addition....I don't think there are any standards.
With DD1 -- I started signing early, around 4-5 months. We loved to watch Signing Time! on PBS and I signed with her all the time. I was also home with her to do such things. At 18 months she knew over 50 signs and was learning them quickly. She has forgotten a bunch since we don't use them. She is also incredibly advanced.
With DD2 -- I sort of started signing around 8 months or so. Mainly more, all done, thank you, please, help. She knows maybe around 15 sings and she is 17 mo. We don't do it with her much...I don't think of it, plus I work and am not home to spend as much time with her. plus they took Signing Time! off our PBS, which SUCKS...its still DD1s favorite show. We have a couple videos, but they are only about 25 min and I forget to put them on and usually dont' like dealing with the hassle of changing them.
Oh, and DD2 is much more middle-of-the-road as far as her skill level, maybe just slightly ahead of average but nothing like DD1. I think it comes down to personality too....DD2 is much more physical and doesn't care much about learning the signs. DD1 loved it and couldn't get enough.
Hope that helps!