how long are egg yolks good for?

The USDA recommends using leftover yolks and whites within 4 days. Bacterial growth = food poisoning. You decide if it's worth the risk.

I use the 5 day 3 day rule. 5 days for things that are raw, 3 days for things that are cooked. I certainly agree with Cynthia, though I'd err on the side of caution and toss 'em.

They are likely at about the end of their useful life. Fortunately, it doesn't sound as though you have too many, so it probably won't be a big loss to toss them. For future reference, both egg whites and egg yolks can easily be frozen. In the case of yolks, be sure to whisk them together before freezing them. If you don't, the membrane surrounding them tends to toughen during freezing and form annoying clots when you do use them.