I generally use Poser 5 to do the images although I'm learning Maya as well.
As to the amount of time it takes, it varies. I generally don't quite know what it going to happen when I start one, it just kind of completes itself.
Since I've been improving in my Poser skills through constant usage, the time frame for most is reduced to just a few hours.
It would be less but I tend to adjust the character or scene, render, adjust again, change lighting, change poses, change background, change this and that, render again, etc.
The actual rendering time is lengthy sometimes but not nearly as lengthy as it was in the past since I recently purchased a new computer. The old one was a 1 ghz Athlon with 700mb RAM. The new one is 3 ghz Intel P4 with 1.5 gb RAM.
That helps.
Sometimes I have someone specific or something specific I want to make. Those take longer. I've spent most of a day on certain images because I wanted them just so!