Signing with your pre-verbal baby can:

In their book, Baby Signs, Drs. Linda Acredolo and Susan Goodwyn suggest you can start modeling signs from birth. After 6 months, however, babies are more likely to possess the ability to remember signs and the motor skills to produce them. You can consider the indicators of readiness below to determine whether your baby is showing interest in communicating. An answer of "Yes" to any one of them means that this is a good time to sign with your pre-lingual baby or toddler. For most babies, this readiness falls between 8 and 12 months of age although sign language can be useful to fill in the gaps of communication through toddlerhood

It depends, but if you begin signing when your child is 6-7 months old it is quite possible that by the time your child reaches 8-9 months he will be signing to you. Some children do not start signing back until they are closer to 12 months old. Typically, they will start with the signs and gestures that involve facial expression (blowing, panting), then whole arm signs (i.e., bye bye, clapping), then hand signs (i.e., more, milk) and finally signs involving various hand shapes and more dexterity (i.e., cat, pointing) Once they have internalized the meanings of the signs and have developed the cognitive and motor skills necessary to sign, they will communicate to you. This process is similar to a child learning speech.

Actually, research states the opposite is true. Using Sign Language with your child can accelerate speech development. Since they are already using language in their heads and putting signed words together to communicate things, once their articulators are able to form the sounds, they quickly add speech to their signs. Gradually, they drop the signs and only use the spoken word. Much like crawling does not inhibit a child’s ability to walk; there has been no indication of a resulting speech delay from signs. Findings resulted from 20 years of research by Linda Acredolo and Susan Goodwyn.