Methinks most of the length is hers, but that she adds extensions for fullness and perhaps a bit more length. By the looks of it, it IS close to HER waist, but then again what's waist length depends on where an individual's waist is. Either way, it looks good and that's all that matters, IMHO.
Many stars wear or have worn extensions.....Christina and Brittney are obvious, but so has Madonna, JLo, Mariah Carey, Cher, (and I suspect) Sarah Jessica Parker, etc....big deal?! LOL
Lastly, I think to compare or even consider Jada's hair or hair care regimen to a "normal" person's is unrealistic in the extreme. This woman is VERY wealthy in her own right married to an even WEALTHIER guy....she has all the time and money in the world to invest whatever she chooses. She has access to the best hair, makeup, etc...products and peeps. Recently Sarah Jessica did a layout in "W" where she was tight and right mere months after her baby was born, but she made it VERY clear that she (like I said about Jada) has all the time and money in the world to invest in herself. She also added that someone that's in NOT her position (where it's apart of her job to be tight and right) shouldn't feel bad if they didn't look that great after a few months. I think the same principal applies to Jada's hair.