I've settled into a 3 week primary time, and 1 week secondary time. In general, if a brew hasn't fermented out in that time, it never will. Provided you do everything you can to ensure a good culture (sanitation, oxygenation, nutrients, a good recipe, etc etc etc), there's really no reason short of an act of God that would prevent it from tuning into good brew.
I take a reading pre-boil to determine mash efficiency (and whether or not I need to add extract to get to my target OG), but other than that, it just complicates things unnecessarily -- in my opinion.
That said, if I'm working with a new yeast strain, I'll do FGs the first few times to get a feel for its attenuation.