My stylist, who I adore, says every 6-8 weeks to keep it from getting raggedy. I'm growing mine out so she does a very slight trim (1/4 inch) every 8 weeks. My hair grows about 3/4 inch in a month, so it's lengthened significantly by doing it this way. After it gets to where I want it, she'll take more off, but I'll probably still stick to the every 8 weeks schedule. (I get mine colored and highlighted. Color every 4 weeks and highlights the times I don't get it cut. That helps w/ cost and with time spent in the salon.)
Couple of inches past shoulder length 3A. Shampoo every 3rd or 4th day w/ baby shampoo; Tigi Fashionista conditioner; back to Condition 3-in-1 mousse . Alternate days water rinse, conditioner and mousse.