How long can a frilled dragon live in a 30 gallon terrarium?

My girlfriend and I have space on our reptile shelves for a 18X18X24 ZooMed Naturalistic terrarium and are trying to think of a cool arboreal animal to keep in it. I've read that baby to juvenile New Guinea frilled dragons can live in an upright 20 gallon long and was just wondering how long you think a baby frillie could live in a 18x18x24 terrarium before it gets too big and needs something larger? Within a year or two my girlfriend and I will have a reptile room to either set aside some space for the frillie to free-roam or to get it a proper adult-size cage.

We will probably end up getting tokays or cresties for the terrarium, but frillies are on my list of dream animals so I was just wondering if it's even a possibility to get one at this point.

Thanks!