I am 35 and in a long-term commitment. There is no way to know exactly how many years you have left. You can get a good idea by looking at averages. The medical profession considers childbearing years to be up to age 44.
Many women conceive naturally after that point.
Menopause and the complete cessation of periods and its related fertility don't occur until the age of 50 or so. So it's not that fertility is gone at age 44, rather that it declines steeply between 44 and 50. The oldest woman to conceive and give birth was in her mid-fifties. Today, women can conceive and carry a child far after that age, with fertility techniques. With these, the age limit goes up to about mid-sixties.