Specific Gravity

How does the specific gravity change throughout the course of a primary fermentation?  And can SG serve as a guide to when to stop the primary and move to a secondary fermentation.  My batch started at an SG of about 20, pH 4.5 and a week later it tastes about ready for the secondary but the SG is still 20 (!) but the pH is now about 3.5.  All this is directed toward getting a successful (fizzy!) kombucha at the end of the secondary.
Thanks for any help or advice.