It's apparent that in this era of continuous innovation in creative media, artists should have great tolerance with the need to adapt. The creative industry in the entertainment media is an ever-changing landscape. While it's an outlet for all passionate artists, it's still a design industry after all. And the language is adaptability, efficiency, and mastery.
Silhouette Over Details
Art directors are not expected to check 3D files. They will always critique a design as an image. Hence, the best way to showcase your design is to strategically compose how much silhouette emphasis is within view. This will be easy for leads to determine how much design has been integrated in a 2D image.
Image Creation Over Asset Creation
While the models in view will potentially move forward to production, we're still in the design stage. Designs will have to be as raw as possible since they will go through numerous iterations.
Don't waste a ton of time fixing UVs, worrying about topology, nor making your models almost close as production ready. At this stage, our only concern is how the final image will look. No more no less.
We're in a time wherein the bar for minimum output quality is constantly being raised. But don't forget that concept art is always design-trinsic. It's always the best design that stands out.
Design First Mentality
Because of how much 3D can offer, it's easy to jump in blender to bash and see what one can come up with in a short time. When in reality, we're actually spending more hours trying to find our design when mix and matching 3D models can only do so much. The most decent that can happen is ending up with a generic design. And that's not a good place to be in as well.
Do not jump in 3D without an established goal in mind. Spending at least 30 minutes of establishing the basic ABCs in sketch already answers 70% of questions that we'll ask ourselves when we're designing blindly in 3D.
So... How much?
As much as you need! It doesn't really matter. Just like other mediums, 3D can either make you slow or fast. But it boils down to how efficient you are as a designer. If you can create the best designs from maximizing what 3D can offer, great! If you can churn ideas with the least amount of 3D possible, MUCH BETTER!
At the end of the day, if you're optimizing time, helping teams, and making art directors happy, you'll be fine 3D or No 3D!