No, she won't gray out. If she had the gray gene, she would have been grayed into a form of dapple or steel gray by this age. Normally, by the time a horse is a yearling (or at latest, a 2 year old) it is fairly obvious that they are graying out. The gray in my avatar is a good example, he's 9 this year. As a foal, he was as buckskin as the horse beside him but by the time we got him as a 3 year old, he was completely a steel gray and has proceeded to get lighter with each passing year.
I would guess that what your mare has is some kind of roaning. What kind is impossible to tell without genetic testing though, there are so many different genes that cause it.
So, while she may develop more white hairs as she ages, she will always maintain her base coat color.