Quote - > Quote - just out of curiosity, what S/px are you guys getting when the render is "done"

for me, I'm averaging like 12-35s/px  I've seen it as high as 305, and I've seen it as low as 2

There are several pieces of information given in the status bar: To the left you have the current render speed (in samples/second) and the average speed, then you have the resolution reached (in Samples/Pixel), and on the right you have two composite values: Efficiency being the efficiency of the raytracing (low efficiency means you probably should change settings), and the last one (EV) is Exposure Value, a value useful for photographers.

I guess the OP meant the definition (in Samples/Pixel, which is the values which tells how much information each pixel has accumulated) of the picture you tend to consider to be "finished", and I guess you meant the speed (Samples/second): At 2 Samples/Pixel your picture would still be a crude mosaic with most pixels still black (since they didn't get any valid sample yet).
But even then, that value seems extremely low: I render right now at 13300 Samples/Second, and that's on an old Athlon XP.
What did you mean?

BTW, to answer to the OP, I start considering stopping a render when it reaches over 200 S/px (the more, the merrier). What I do, is zoom in (you all know Lux has a zoom feature, don't you) and check for noise and spots which need some more cooking; If there aren't any, I stop it.