Angie - when it turns rancid you will know it - without a doubt!!!!! It will just smell and taste old, and well, rancid! Hey, and while you put it on your face put it on the back of your hands too. I missed my chance a looooooooooong time ago to do this! lol
If you want to help keep it longer make sure you keep it in a porcelain bottle so no UV rays can penetrate it and keep in a cool place with a lid on it. If you keep a pouring spout on it just take a piece of aluminum foil and cover it tightly.
"Count yourself...you ain't so many" - quote from Buck's Daddy