My freezer takes about 24-27 hours. You can eyeball it. There are usually a few bubbles floating around just below the freeze line giving you an idea as to how thick.
PS - I just use cold water straight from the tap. I'm in Chicago and we have pretty good water.
OK gentlemen, here's how it's done (or how I do it anyway)
1. Fill lunch cooler with water and freeze until approximately 2 inches thick. This forces directional freezing from the top down.
2. Remove block from cooler and shape into slab.
3. Score slab with a serrated knife into desired size...