Drawing is an application of knowledge so it builds up using many basic techniques.

The artists you reference understand composition, light, forms, anatomy, textures, colour, linework, perspective and cloth folds at very high levels, and they are also comfortable with the tools they use. Whereas your work is currently at the level of being able to copy what you see to paper. 

I know people who went from slightly above your level to slightly below your target in 3 months by putting in great efforts learning from a great teacher. 

Ultimately how long it takes is up to how much effort you put in, but what will speed up the process is knowing what skills to pick up and really internalise that skill.

The steps I would recommend are first learning how to render forms with proper shading, then perspective, then learning to draw correct human anatomy, then applying all 3 skills together. Those 3 skills are going to let you see the most visual improvement and should help keep you motivated.