1) It depends on your hair. I think that it depends on the strength of your hair -- in my experience, thin, fragil-ish hair (not talking damage but just diameter, density, etc.) just likes ... other hair. Sounds like a riddle but it's not. I remember in school that when everyone chopped off their hair to get layers or a Halle/Nia style, the people who experienced a lot of rapid growth all had thickish hair to begin with. Those who had thinner hair (like moi) either, stayed with short hair or had a really hard time growing it out. Conversely, those with a blunt cut (one basic length with bangs or an inch or two max cut into the crown for shape) had no problem growing hair out. I think that when thin-ish hair is cut short and not "protected" by other hair, it is vulnerable. Ironically, these are the people who really want to cut/layer hair to make it look thicker -- beware the allure of illusion.
2) Depends on your regime. As everyone has said, it also has a lot to do with care and how you handle the transition point. Direct heat is just out -- leave it alone until you feel comfortable letting the relaxed hair go, if ever. I would suggest roller setting, gentle combing only when conditioner is in hair, etc. -- you know, the basic HHC routine that every should know by rote by now.
3) A suggestion for easing the transition with or w/o BC. This might be the most important piece of advice (IMO): soften your new growth from the inside out -- e.g., by taking Horsetail or Bamboo Silica, MSM (if you can tolerate it, I can't).
4) My "testimony" lol. Hope that helps. I wore weaves for several years, had about 10" of new growth, and then ... as a result of a weavecare set back and take down tangle debacle - had to cut it all off and more ... my hair has never been the same in terms of length, thickness, etc. Granted, I'm in my 40s so my growth rate may be slower than yours and, believe me, it is far (far) better than it was because of info/techniques learned here but ... it was never the same after the BC. Just sayin' It is tempting, I know; because even with my experience and hindsight, those scissors beckon every now and then.