The first mention of a special time piece in the Weasley home is from Chamber of Secrets, page 34. However, this appears to be a different clock.
The first specific description of the family clock is from Goblet of Fire (pages. 151-153)
It appears again in Order of the Phoenix (pages 471-472)
Finally it's described in Half-Blood Prince (pages. 85-88)
This last description seems to conflict with the more detailed description in Goblet of Fire. In GofF, the clock is a Grandfather clock. In HBP, it is small enough to carry around. The GofF description seemed geared at giving ambiance to the house. The HBP description seemed more intertwined with plot and character. I chose to follow this one more closely and finally settled on a mantel style clock for my own design.