Babies can learn simple signs as young as 6 months but its safe to say that baby may not have the hand control to be able to sign back to you until she's older, about 8 or 9 months old maybe. As for duration of lessons, as the saying goes, practice makes perfect so whenever you can, you should practice with your child. It's just like any other language, the more you practise, the more fluent you get :)
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Mama of 2 cheerful daughter