My son was 18 months and only speaking like 2 words. not even MAMA. We started speech therapy through the county, and I started getting serious about signing.
I had always wanted to sign with him, but I didn't spend enough time to really learn much. SO when he wasn't talking, I started looking for classes. We took a paid class, when to a 3-day free class at Maryland School for the Deaf, I looked for signing things at the library (books, DVD's), participated at library signing classes (Kathy McMillan is our favorite when she is there.) We did up to 2 classes a week, and watched signing videos every day, and I read some adult signin stuff on my own.
He was signing and communicating in no time. He was putting multiple words together using sign within months.
It was so beautiful to be sitting in a car stuck in traffic and see him sign "cars, lights, fast" too me. (The lane going in the other direction was not backed up, but ours was.)
His big signing ureka, came a little before 2 years old, and then all of a sudden he started building his verbal vocab too. Slowly at first, I think he had 5 words by the time he was 2. But by 2.5 he was talking enough the speech therapy ended. By about 34-35 months he was talking in run-on sentences... and a vocab I can't count anymore.
He is still not as understandable as he is suppose to be. The speech therapist said we can restart under the education program if it doesn't pick up well enough.. I hear him improving all the time, so I am waiting until time to apply for 4 yr preschool at Twin Ridge, and if he's still behind a little, I may have him evaluated.
Even our speech therapist said that my son was the poster child for how much signing can help with speech. (She even thought signing did more for him than the speech therapy.)