He should grow about 18-24 inches like most Blue Tongue Skinks. Meraukes tend to be the longest, while Northerns tend to be the biggest. The 30 inch one (If your talking about the same one I'm talking about) wasn't 30inches but 28inches, "creeping" into 30 inches. None have gotten to 30 inches but a rare few have gotten close. I would recommend looking into these sites:
bluetongueskinks.net/forum
bluetongueskinks.com/forum
P.S - Yes, these are two completely different sites.