Since mine are still only about the size of my hand I can't say from personal experience but have talked to people that say they get about 20".
They are pretty motion orientated and it took some work to get mine to eat frozen foods. I've yet to get mine to take any prepared foods. They will inhale feeders and have huge mouths.
I think they tolerate most other fish but if it can fit in their mouths it's over. They're not very active fish.
If this is the same fish I had as a marbled sleeper it got to about 18" before I sold it, It grew quickly, hardly moved, and swallowed whole anything up to 1/2 it's size. It would suck up fish as big as it was and then spend a couple days slowly swallowing the parts that stuck out. Looked pretty funny.
How long did it take for yours to get to 18" guppy? Yeah we're talking about those nice big sleepers. Uh maybe I won't be letting the jd go in that big tanks despite getting along with the clown knives....the marbleds are going in there when they get big enough to not get chowed on by the clowns.
Less than a year from around 8", it ate bullheads, channel cats, sunnies, lm bass, a p. cichlid, a couple rb pirahna, and everything else that I put in with it. I got it free after it had been returned to lfs for eating peoples fish.