Well there are different phases of hair growth, and different regions of your body are "programmed" to be in the growth phase for different periods of time. Yay genetics, but nutrition and hormone levels can also play a part.
Your head can allow for years of hair growth, but I would imagine that if the turn-around rate of your hair growth was faster, then it would reach a certain length then stop. I don't know much on the topic so this is just my speculation.
Locks of Love is a great program, I hope you'll be able to donate. In 2003 I donated a 12" ponytail and still had hair down to my shoulders after I cut it lol. I'm back to leaving my hair long again (about 3" from the top of my jeans) so the next time I decide to cut a good bit off, it will most likely be at least 10" so I can donate it.