Not sure if I'm officially midback yet (haven't checked), but I was APL last year from May through July before I started transitioning to BSL. I was full BSL by Nov '06. I think I had a major growth spurt last summer because I trimmed about 3 inches between May and Sept '06 and still hit full BSL by Nov (6 months post APL). Anyway, I trimmed another 1.5+ inches again in February '07 and haven't trimmed since. But that last trim kept me at BSL.
I said all that to say that had it not been for all the trims, I would've been midback in February or March of '07. Which means, it would have taken me about a year (more or less) to get from APL to MBL. I'd actually be close to waistlength right now had I not trimmed.
But anyway, can't cry over spilled milk.
My hair grew and retained from drinking lots of water, low manipulation, protective styling, light heat, and regular deep conditioning. I know trims don't make your hair grow, but my hair seemed to thrive when I was getting regular trims? This year I made the decision to only trim once a year. Since I already trimmed in February, my next trim won't be until May '08. I chose May because that's exactly 2 years from the month I was 100% natural and APL. Hope I didn't confuse anybody!
ETA: My growth rate is less than average. I think I only get about .35 a month.