You can make a material so complicated that it probibly can't be rendered in a acceptable time. I was thinking about that later with nested textures. I try never to go more than 2 layers deep. Going sideways if you know what I mean is a better idea. If you're just making bark I tell ya what I do that's fast and painless. Go find a nice picture. Drag it into photoshop and make it seamless. Import it into a texture and then import it again as the bump map. Done, fast, simple. You can even grow moss on it with mixed materials for added effect. It looks real because it is real. 512 X 512 size is excellent but you can even get away with 256 X 256 size pictures. The other thing is you don't have to wait for the preview to finnish. If you think it's good enought just click ok and work on you scene again. It don't matter if it finishes. The only time you need it to completly finnish is if your going to save it and you want a nice preview picture.