Okay, I put in Hugo 3D (no pop-out gags).
At 8 feet, and assuming the screen is perfectly perpendicular, I can raise my head a little above or below the top/bottom edges of the screen before detecting any crosstalk at the further edge. Approaching the screen with my eyes at the level of the screen's center, I can approach as close as ~ 3feet before seeing crosstalk at the top or bottom edges. Actually, my TV is wall-mounted a little above eye level and tilted down enough to have my eyes perpendicular to center screen at seated position. But I did the best I could. It helps that my screen is very glossy and I could see my reflection if I opened the blinds.
Well before 3 feet, the 3D becomes uncomfortable to me. It's there, but fatiguing; progressively harder to focus on. Without watching for an extended period at various distances, I'd guess 6 feet would be my limit without getting a headache. YMMV.
I guess you could call that best case scenario for 3D, crosstalk aside. Let me give you a worst case scenario:
I capture 3D from cable with my Hauppauge HD-PVR over component. It's the only way I can watch them actually, because strictly speaking you need an HDMI connection for that, to manually put your TV in the proper mode. My HD cable box has component, no HDMI. Which is as I want it, because my HD-PVR would then be useless without component. Yes, we're talking SbS 1080i, which is less than ideal on a passive set. Still looks pretty good.
There's one capture I have, 3D Sun, which has very aggressive 3D, lots of pop out gags. That one is very fatiguing to watch even at 8 feet. At times, it takes some seconds to adjust one's eyes and see the 3D. In the interval, there's double-vision, like one took off the glasses.
In between you have 3Ds with moderate pop out. Off the top of my head, and this might not be the best example, there's Imax Space Station. It has a few scenes with fairly aggressive pop out. My eyes can handle those okay at 8 feet.
All that aside, and I'm sure you know this anyway, but you need a balance between immersion and 3D. They're not mutually exclusive, but 3D effect will tend to be better further out than immersion, for which closer is better, up to a point.
Since no one else replied yet as I'm typing this, here are a couple representative links, which I probably read when I was going through this process of figuring out proper viewing distance:
I hope that helped you some. Good luck.