The short answer is that you'd need close to 50% equity or more to justify calling "any" bet and those situations are pretty rare on the turn, let alone the river. About 15 outs on the turn would justify that while about 23 outs or half the deck would justify it on the river. You'd be hard pressed to find a feasible situation where you have 23 clean outs

The only feasible situation that I've personally had come up similar to this is when you put your opponent on an underpair and you are drawing to a flush, open ended straight, AND both your overcards.

In reality, stack sizes, your current equity, your actual outs, and implied odds rule the day