How Well Words Can Be Understood By Parents
By 18 months a child's speech is normally 25% intelligible
By 24 months a child's speech is normally 50 -75% intelligible
By 36 months a child's speech is normally 75-100% intelligible
Source: Lynch, J.I., Brookshire, B.L., and Fox, D.R. (1980). A Parent - Child Cleft Palate Curriculum: Developing Speech and Language. CC Publications, Oregon. Page 102
How to Build Language and Literacy Through Powerful Conversations
This is an excellent paper that describes methods for parents to increase and make more productive, their communications and other interactions with their children. It is definitely worth a read.
Source: By Janice Greenberg, Director of Early Childhood Education Services, and Andrea Lynn Koohi, Staff Writer, The Hanen Centre