I don't think it's about how much you wash per-se. Most of us on this board dont believe in the "dirt grows hair" philosophy so we wash pretty often. I think the key is manipulation and how much you use heat. When hair is that short, some people resort to keeping it straight and that means a lot of heat.
When my hair was almost BSL, it grew ridiculously fast because I could bun and because I didn't have to use heat as much to look semi-decent. But I had a setback recently and I may have to cut my hair to neck length or right above shoulder length. I'm worried that I won't be able to bun or do my twistouts because of the dramatic loss of length. When hair is that short the only things I can think of is using weave/braids to help it grow or rollersetting it. Neither of which I am interested in. I would be curious to hear about other protective styles which can be done or SL or shorter hair.