A rule of thumb we used in the stewarding department at school was reboil after the first three days, and if it gets to be time for a second reboil, throw it out. And of course, if you dip into the container to take any out, make sure you use an absolutely clean ladle or spoon. (You don't want to introduce anything else that could go bad.)
Stock also well in the freezer -- just make sure it is well-sealed, because it too can get freezer burn. If it's protected from air, it can keep for months in the freezer.
