When I see myself some guys aiming for 10" or 11", I can't help myself but think "wtf ?!!", but I guess every man has his own reasons for his own goals (they can be very tall, and want a long dick to get along with it for example), so I respect that, and I think the most important is to know what you're doing and why you're doing it. It's a very intimate and personal thing after all.
Munto is totally right too, I guess if your loved one doesn't stop you, you could basically go on forever ... My wife already thinks I'm big enough for her, but the thing is I can't quite reach the end completely yet on all positions (mainly doggy). When she's totally aroused, she can be quite deep at times, so now you know what is my real goal behind all this
It'll be 9" or whenever it feels long enough and comfortable to be able to completely "feel" (and "fill") her until the very end, without hurting her of course !
Once I reach this, then, I'll only focus on Girth, and there, I'm afraid I may go very very far since I love so much to be THICKER and THICKER....