My scene Unreal take on classic has 2300 meshes with all walls, roof and floor @ 1024 Resolution with other meshes at 256 resolution minimum. It took 22 hours on 2 x i7 4930K 32 GB RAM each. When you think what you are getting in the end - Unlimited camera angles for still images and animation, 22 Hours seems like a breeze.
You have to experiment with different values, and avoid Indirect Lighting Quality = 10 (For me it always worked between 4 - 6).
That’s what we are targeting here - Balance between quality and rendering time.