Do I need six month H1-B to get my wife a H-4?

Dear subject experts:

I am trying to apply H-4 for my wife in Guangzhou. My H-1B is going to expire on Dec. 1 2007 (5 months from now). Is there a rule that H1-B has to be more than 6 months so that my wife can get her H-4. If chance is small or impossible, I might not apply H-4 for her.

Please share you knowledge or experience with me asap.

Thanks