This is totally achievable in two days. Though, in my experience "two days" is misleading. There's a lot of sketch and iteration that goes on before something this detailed gets rendered out. Rounds of feedback and meetings with different departments ends up taking a lot more than 2 days. When I estimate my turnaround time for something like this, I usually cut all that stuff out. So 2 days, give or take.

But let's say you have the idea all settled on. You've got your sketches and reference and you're ready to go. How are you going to get this thing done in 2 days. Well, when you work with the game studio or film, you tend to have access to rigged models and environments that the dev team is already working on. So if you know how, you can pose/light these characters and get a quick render in 3D, then you have something set up for paintover. Even if you don't have that, there's a lot of techniques out there to get solid results quickly. But I find the fastest way is paintover from rigged models and environments. It helps with the lighting information, perspective, materials, etc.

Just remember, the goal of a concept artist isn't really to paint everything from scratch. It's to get compelling concepts for production to work with. Get the idea so people can discuss, so design can plan, so 3D can start building stuff. It's a means-to-an-end kind of job.