About how long did it take you to really dial sketching down?
There are many different sketching styles, so its tough to really put a number on it. However, I started drawing in general at around 10 years old, discovered product design when I was 17 years old, and product design sketching didnt really ‘click’ for me until I was 21 years old… So I would say about 10-11 years before I felt confident with my sketching?
Do you feel like you have to learn to draw each object or can you just draw any object at any time?
There is always going to be a learning curve. You need to study, break things down, and analyze why things look and work a certain way. However, if you know the basic geo forms you will at least have a starting point because even the most complex shapes and objects are simply a combination of basic forms: sphere, cone, cylinder, cube. If you can draw these well and at any angle, youll be able to get the hang of new object quicker than someone who cant draw the basic forms.
Rendering, How long did it take you to learn? Marker? Digital?
Still learning to this day… I’m more confident in sketching than my rendering skills. It really comes down to do you understand how light and shadows work. I guess I still don’t really get it. 
Does anyone else get freaking frustrated on insta by looking at those super amazing digital renderings some godlike automotive designers are producing these days? Seriously.
No not really. I dont get frustrated by other people, I get frustrated when its not working for myself when i’m practicing. But, you need to push through it, the more you dont get something, the more important it is to figure it out before you move onto something more difficult. Tons of practice makes perfect.
Rate the importance each, Digital and Manual Sketching. Which do you find more difficult?
Honestly, I think they are equally important. But its really less about the medium and more about do you just know how to draw/sketch? Once you know how to sketch, the only real difference between the two, is learning the medium. In manual sketching, learning how different pencils and pens behave and how to best utilize its perks. With digital, learning the various softwares you want to draw in, their pros and cons, what the various tools do, etc. The difficulty between digital and manual sketches is in learning the tools. If you don’t understand the basics of drawing, sketching, perspective, and rendering… your sketching is going to be difficult and crappy no matter which medium you use. 