I have a jar of organic tahini which expired one year ago.  It smells good and tastes ok. I don't know how it was stored, it was however always sealed (but already opened).  Is it safe to eat in large ammount ? 
To the best of my understanding, Tahini is a lot like honey in this respect. It has no moisture in it, and will therefore suck the life out of anything that tries to land on it. Theoretically, as long as there's no water there, it'll basically last forever.  I usually keep mine in the fridge, not for temperature control but to keep the humidity away. I do live in a swamp though. I'm still using some I bought about 7 months ago, and it's fine.  Once it's been prepared (i.e. mixed with water) it's good for 3-4 days. 