All cells in our body grow through a cell cycling phase, and then an aging phase where they change and finally a phase where they do not renew. No cell in our body does this as obviously as hair.
When you ask how long your hair will be growing, to some extent it depends on your genetics, your nutrition, and how good you are to your hair so it will continue to grow. Or not grow. At age 40, we start loosing the hairs on our head, literally.
The hair follicles literally just die off if you don't do something to make sure some last, they cease to be there, we start to drop from a max of about 100-150 thousand hairs on our head , to, well, lets just say "fewer." And the hairs you have left will start thinning and narrowing as well.
Hair thinning is noticeable to you as your hairs just start being farther apart from each other, and suddenly your hair stylist is sending you in to get your hormones checked, which likely is not going to give us a solution to reversing this.
Although you can probably name some older women with a beautiful head of hair, about 70% of women by age 80 report significant hair loss. And yet not all hairs stop growing.
Oddly, some hair on the body, as we age, become more noticeable. Under the influence of the male hormones the fine hairs on the body start to coarsen. So hairs that might have been downy, soft, basically so nu-noticeable you never noticed, they now become thicker and darker and seem to have "appeared" out of no-where, such as arms and legs. Rather than specifically age related this hair change is more years from menopause related as it is linked to your total hormone exposure.
It is worthwhile, for any hair changes, increasing or decreasing evaluating your hormones. Hormones have a role, and it's more linked to male hormones, but exactly which and what levels, that remains to be worked out. For young women treatment of hormone imbalances may indeed cause the hair to grow.