There absolutely no answering this. I have a beed with a trun of 15cm, which took 15+ years to develop. Doubling this will take 15-20 years of growth at least. I also have Linden tree seedlings, which have more than doubled in thickness over this spring. It all depends on the species and the size it already has, and the skill you have in growing plants.
As for "nebari characteristics".. Same deal. If you manage to keep the roots very close to the surface and get good growth, young plants may develop horizontal rootflare very quickly; Again: I have seedlings of which I clipped the taproot. After one season of good growth, with root-cleaning & reorganising twice in the season, I managed to get a decent rootspread, which after another year groth will result in clear surface roots. On the other hand.. I have an 15yr old acer with a stubby heel to the trunk and I expect getting a decent nebari may set me back another 4 years or so.