An opened jar of tahini will taste good for about 4 or 5 years if stored in the fridge, covered. If stored in a dark place at room temp it should last about 2 or 3 years without much of a flavor change. In the fridge it will be safe to eat for about 15 years, about 6 years unrefrigerated. Obviously if you see stuff growing on it, don't eat it. The first thing to go will be the oil will turn rancid. It will taste rancid, but it won't harm you. There is almost no moisture in tahini, so nothing will live in it. The problem of it eventually becoming unpalatable is due to rancidity, not bacterial action. Of course, always use a clean spoon when taking tahini from the jar. Transfer to a glass jar if it came in a metal can (the metal from the can will give the tahini a metallic taste, as the metal slowly dissolves in the oils, but it will take about 60 years before there is enough metal in it to harm you). Make sure there is no food or moisture on the spoon you stick into the jar of tahini. Keep it covered of course.