Noone knows exactly, except Reduz, it all depends on him :). I think that even with current engine you could theoretically get close to visual quality of other engines, but the bottleneck is the awful performance and lack of features that make life easier. Also another problem is that no AAA studio uses godot, so noone is even trying to create photorealism.
Reduz is currently working on implementing Vulkan and improving the 3D rendering pipeline, which will surely make Godot more competitive 3D engine. I believe that 4.0 will satisfy the basic feature needs for creating a decent 3D game, which will probably attract more people to use Godot for 3D. More users = more nice looking projects.
But I think that there is still at least one year to go until that basic feature set will be satisfied. So don't expect too much in the "near" future :) But Godot is on a good path to become the "Blender" of game engines in next couple of years (best open source alternative - which can be used for big projects).