When do you draft a quarterback if they fall?

So I know that the general consensus is to wait on a QB because the points between QB1 and QB12 is much smaller than the difference between RB1 and RB12 or WR1 and WR12. However, usually in most drafts there is an inevitable drop of a talented QB. My question to you guys is when you know to take them aka their "value" is good enough to take instead of bolstering RB/WR depth or even getting a good WR3.