I tried seriously stretching a few times, but my experience has shown me that 12 - 14 weeks is a max. HOWEVER, my hair grows at different rates - sometimes faster, sometimes slower. If my hair is growing quickly, then I may have to realx a lot sooner than I would during a time when the growth is slower. Given that fact, I never set specific time goals for stretching (like telling myself that I'll stretch for X number of weeks) - I just go by what my hair will tolerate.
Additionally, since I self relax, I don't want SO much new growth that it would cause me to have a hard time relaxing w/out tangles, getting sloppy/overlapping, or running out of time before I can smooth my hair properly.
I definitely can say that once I stopped relaxing every 6 - 8 weeks, my hair did start to grow in thicker. I'm 6 weeks post-relaxer now and I can't IMAGINE realxing at this point. Yes, the new growth is present, but I keep my hair clean and VERY moisturized so the growth is very manageable.
So, to sum it all up, I am always impressed by the all of the reported hair successes from all of our forum members, but the bottom line is that I do what's good for MY HAIR.