I would wait until she is looking at you and "communicating" before I'd respond to the sign. You're teaching her to communicate, not just do signs, kwim?

We taught DD2 one sign for boobie (pat your chest) and another for milk as they get to the age soon enough when they are having milk from a sippy cup. It took my DD until she was a bit older to properly get the idea of signing, now she's picking them up really quickly and using them very intentionally. Your efforts now will pay off soon enough!