I did the big chop in July 2005. From my pics, it looks like, at that time, my hair was SL when stretched.
SL--July 2005
APL--March/April 2006
Barely making BSL (very top of my bra strap)--July 2006
BSL (bottom of bra strap)--October 2006
WSL--June/July 2007
So, it looks like I went from SL to WSL in two years. These are stretched lengths, though.
Things that helped me retain might be: protective styles every day (buns and updos), no heat, no trimming, frequent co-washes (usually daily, but sometimes less over the last couple of years), no combing or brushing EXCEPT with conditioner in the hair and only with seamless combs, and using natural butters and oils.