2x bw DL is where things start to get interesting. In fact, my DL generally sits right here until I start doing things with it, but then again, I'm running 265# or so.
I really think that the vast majority of folks can get here in a reasonable amount of time. And I also think that for many it's an unconscious fear of the weight that keeps them beneath it.
Those with power-biased athletic backgrounds tend to be less afraid of the heavy weight, and will get there more quickly.
DLing around 405, if you've never been there, can get a bit scary. I remember that from my own ascent.
For the record, it took me 6 weeks to go from 315 to 405. 2 weeks to go from 405 to 445, and maybe 6 months after that to go to the big 495 (5 plates a side). Then it took me 2 years to get to 570, and that's as far as I got. I sit at right around 535 until I push it. I really need to improve this lift.
You need to practice the lift itself, but you also need solid, grinding posterior chain building exercises like the GM, SLDL, and RDL, and getting the quick lifts up (PC, PS, HPs) is a nice trick too.