At perfect (chemistry lab theory) room temperature, 4–6 hours should still be safe. In reality, hummus that's at an outdoor summer picnic — gosh forbid it’s a bit muggy or humid out — has been dipped into. By the end of that 4 hours (even within the time frame) the hummus has already started heading southward fast. Depending on how hardy your stomach is, you might be fine — but if you have any stomach sensitivities…be prepared for the worst, hope for the best.