Speaking of torches: when it says mobs don't spawn "in the dark", are there any specifics to that which anyone knows? For example, if I place a torch (above ground), how far will it "protect" (or I guess more specifically, how close do torches have to be together to consider an area "lit")?

Basically I'm trying to light up a large area and I'm not sure how many torches to use. For the most part I've just been placing randomly in the darker looking spots, but I did see one spider in the area so I'm not convinced that I put in enough....