Hi! Welcome to the board!
In a nutshell, it's good to stretch your relaxer because it gives your hair a break from the harsh chemicals in the relaxer. These chemicals essentially break down and destroy the protein bonds in the hair, which is what makes it straight. Unfortunately, with the protein bonds goes strength and resiliency, which is why for many people with relaxed hair, breakage and thinning of the hair can become an issue. By waiting longer in between relaxers, your hair has a chance to rest and regain its strength before undergoing the process again, thus decreasing the likelihood that your hair will break or thin out from using relaxer.
And no, it's not really a myth. When you get new growth, the place where the new growth connects with the relaxed hair is known as the line of demarcation, and it tends to be a weak spot in the hair, just because of the disparate textures. Breakage can occur at the line of demarcation if it isn't given enough moisture AND protein to maintain its integrity. Breakage there can also happen (and I think this is the most common reason) if the hair is simply handled improperly. That is, if it's being brushed or combed too much or too hard, if it's being washed too roughly, or if undue stress (like from ponytails or buns) is being placed on it. Most people incorrectly assume that not relaxing will make you lose your hair because they don't know how to properly handle their two textures, so they get lots of breakage. But the truth is that if they knew what they were doing, they would be just fine. As with most things, stretching takes some technique and a good plan. With some knowledge, patience, and the proper technique and products for your hair, stretching should not be a traumatic experience, nor should you end up with less hair than you started with.
With stretches, you want to start out slowly and then increase the weeks as needed. Meaning, I don't recommend that you start out stretching for 6 months. If you've never gone past 4-6 weeks, then start out trying for 10. If 10 goes okay, then try for 14. If 14 goes well, try for 16, and so on and so forth until you find a timeframe that works for you. Some ladies stretch for 6 months often, some don't go past 12 weeks. Everyone's hair is different, so don't feel pressured to leap into something that you don't think you can handle.
IMO, stretching relaxers is one of the best ways to keep relaxed hair thick, healthy, and happy. There are quite a few threads on it, including the original
Relaxer Stretchers Support Thread here:
http://www.longhaircareforum.com/showthread.php?p=9920062
and the
Long Term Stretcher's Challenge here:
http://www.longhaircareforum.com/showthread.php?t=356989
Both of those threads are full of really good information...they are lengthy and it can seem overwhelming to read them, but I think they'll be helpful to you.
Phew, sorry about the novel.
I'm fairly new (joined sept of last year) and I plan on stretching for 3 months which is a first for me since I usually go about 8 weeks
, I would love some help and support but the length of the current thread is a bit intimidating. 
and let's try to fight the urge of relaxing too soon together!!
(lol)
I'm four weeks post and I'm hoping to do my next touch up in June at 24 weeks! I'm going to need prayer! 
