Time required for "scratch coat" to dry

We want to re-brownstone our stoop and have gotten different suggestions about two things:

1. Amount of time necessary for the scratch coat (cement) to dry before applying the brownstone.

2. Need to re-do the scratch coat at all. Currently the brownstone has come off in patches, exposing the grey cement beneath. One of the contractors has said that the concrete below seems to be in good shape, so it may not be necessary to re-do it at all. Is this legit, or simply a way of cutting corners?

We’d appreciate any advice on either topic.