Thanks Michael!

The full image size is 2400 x 2400 pixels, and I let it render for around 4 hours on an Intel Core i7 970 Hexa Core (6-core) CPU PC (3.2 GHz, 12 MB cache).

Cycles is faster using CUDA GPU rendering, but although I've got a fairly good GPU card (ASUS GeForce GTX 480, 1.5 GB GDDR5), the scene is too complex and the render resolution is too large to fit in my graphics RAM when using Cycles.