@anssi said:
What is meant by "thick glass"? Must it be two parallel faces close to each other, or must the sides be modelled too?
If you have a couple of minutes, you should check out the SkIndigo tutorials for version 0.9:
http://www.indigorenderer.com/joomla/forum/viewtopic.php?t=2317
They are interactive skippies. 'Thick Glass' would be glass that is modeled as a closed volume. I think for windows, you would not have to model the sides to get the correct result (not sure though). It is pretty easy just to push/pull your windows 1/2" in order model the window correctly. However, you can also avoid this step by using the 'Thin Glass' material. It works pretty well for windows but I would not use it for glassware. If you use 'Thin Glass' make sure the exponent (glossiness) is set to 1 million so you get near-perfect reflections. Lower values will result in a glossy appearance.
If you use the 'Glass' material preset, you will get perfect realistic glass, but the glass must have thickness.
I think your render was slow because of the high resolution and the large amount of windows that cause the light rays to keep bouncing back and forth. This is also the reason why materials that are completely white tend to stay noisy much longer (lots of light bouncing)