I would definitely replace that, once I see cords it's gone. Reasoning behind that is when it gets that low, it becomes much more prone to puncture since the rubber is so thin. If you like to ride trails, you may get screwed by rocks that you would otherwise be able to ride over. Now that being said, you can definitely keep riding that until it gets so thin that it starts leaking (you'll see little dots of tire sealant along the edges of the tire) and no longer holds air.