@Bleuyteuy I don't know too much about porosity but I trim my hair about every 4-5 months so that could be what you need.
I'm only 5'5" so I would imagine my hair would be about 3 inches shorter than yours at waist length. My hair is cut into a v shape too so I figured when I got to waist length I would maintain it there until it was all even, but knowing me I will probably keep growing until it gets as long as it can.