That is the worst conversation. You have to admit you can't do it, and you have to force them to evaluate what is worth what.

Of course, part of that evaluation is you. Almost unavoidable.

They have to admit you are not an infinite resource and that they are asking for more than is realistic.

The very worst is by the time this all happens, they have most likely already pre agreed to just get it done, or they themselves were told to just do it.