Just to show it's not simple, DS had fewer than 5 words in regular use at his 22 month check up! He was talking in 5 word sentences by 26 months, though; those 4 months in between were interesting.
My daughter was progressing at a pretty normal rate initially, speaking her first word at 9 months, speaking several words and using a few signs at 1 year old. By 18 months she was using over 100 and asking "Where's the bubbles?", and by 2 she knew at least a thousand words and told me "Buttons the bear broke his arm just like Woody." Which was true, and which the pediatrician told us was not normal.
Within a few months of her second birthday I began worrying that she wasn't learning several new words a day that she was capable of, simply because I didn't expose her to several words a day that she didn't already know. I had to make a conscious effort to diversify my language and include middle-school and high-school vocabulary words in my conversations with my 2 year old. I intend to send her to preschool when she's 3, and I'm beginning to expect that she'll occasionally use words the preschool teachers may not know.
I guess this sudden acceleration is somewhat typical of children raised to be bilingual. I can't comment on her Chinese proficiency, but perhaps her "normal" early language development was a delay of abnormal development: as if her ability was 30% advanced, but the exposure to 2 languages caused a 30% delay or something to that effect. Or maybe every kid just takes their own path.
