Once you start using baby sign language consistently, it may still take awhile for your baby to sign back to you. The older the child, the faster they usually catch on. I began signing to Nate regularly when he was about four months old. (Four months is on the early side and baby sign language can be beneficial even if you start much, much later.) His first sign, MORE, didn't come until he was about ten months old.
An important thing to remember is that you should always acknowledge your baby's attempts at signing. The more the child hears the verbal word in connection with the sign, the faster their vocabulary will grow. When your child knows you understand them, and knows they are communicating effectively with you, the more confidence they will gain in their signing ability and the more incentive they have to sign.
In the video below, I demonstrate what not to do. I was so focused on trying to encourage Nate to crawl, that I pretty much failed to acknowledge any of his attempts to tell me that he wanted MORE of something.