Hi there....18" is on the large side, they could be anywhere from 12 -18 or more [ rarely ] at 6 months. He won't grow that much more now, probably another 3-5" average by a year old. And some grow in length very quickly [ like yours ] but then don't get much longer than that. But my guess would be that he'll end up around 20" -21".
Since Rex had a nibbled tail end when we brought her and Puff her brother home, and had lost maybe 5-10% of the length of her tail, I never bothered with measuring VTS (vent to snout) , or TTS (tail tip to snout) lengths. I focused on body weight as my indicator of growth.
For Puff (boy):
6mth old = 176g
12mth old = 396g
1.5 yr old = 536g
2 yr old = 685g
For Rex (girl):
6mth old = 221g
12mth old = 409g
1.5 yr old = 513g
2 yr old = 720g
hope that helps.
PS : from about 12 months old they had a insect per day + greens/veg/pellets daily, and neither has ever brumated.