The earlier case of a Royal Gazette announcement apparently published before it was approved made me program a little statistics to check the time elapsed between the date of approval and publication in those announcements in my XML. Besides this helped me to find a few entries where I mistyped one of the dates, I also confirmed the above mentioned case was the only one so far out of 3500 announcements I have processed already.
As a mean value, 42.7 days elapse between the date it is signed for approval and it is published in the Gazette; yet most often it's exactly three weeks between the two dates. The longest times elapse for municipal announcements,with the upgrade of subdistrict municipality Sattahib to a town taking almost one year (Volume 124, Issue พิเศษ 184 ง, Page 2). It was signed on December 12 2006, published on November 26 2007, and took into effect on January 12 2007. Also, the creation of subdistrict municipality Prasat Thong took 360 days.