The following is a timetable of 'an average baby' from the book I have:
3-6 months: While babies are too you to begin any serious signing, this is a good time for you to put your baby in training and jump-start the process. Plus, it's a great time for you to practice what you're about to teach.
6-8 months: Your baby's memory skills are growing daily. If you begin signing when your baby is 6-7 months old you will probably see results about 2 months later. This is based on a high level of commitment and consistency.
8-10 months: Your baby's coordination skkills have improved dramatically and he/she will be able to produce simple signs and begin to grasp their meaning. If you start at this stage, you should see results in 6-8 weeks.
10-12 months: If you introduce signs to your baby at this age, you should see results in 2-3 weeks.
12-18 months: This is a prime imitation period. Watch what you say and watch what you sign. It could be picked up instantly, although the realization of what you're doing and why you're doing it, maytake a few days to a week.
Signing Variables:
Baby's age
What else is on their minds? (trying to master other tasks)
Baby's mood and interest level
Your level of commitment
Your consistency level
Family's level of involvement
We're only signing a few things right now, mommy, daddy, bottle/milk, bye-bye, and love you. We'll add in more around the 6 month mark things like hurt, diaper change, more, food, etc and then build from there.
I don't think I started with DD1 until 7 months and we only learned about 10 things total but it really helped a lot. I'm starting early so I can get DH and DD1 on board with the 'program'.