We cast a concrete step today in between my lower ground floor and my upper ground floor. It is approximately 19.5cm in height (riser height)  and 29cm in depth (going).

 

It has a 110 waste pipe running through it which we are using to supply air to our wood burning stove.
 

here is an approximate cross section:

 

 

It is rather long, as it runs along most of the width of the entire house, about 7.75m I would estimate. 
 

How long before I can lay a floor finish on it? We are planning to install 20mm terrazo on it (which is highly porous and cannot be sealed until after it has been laid) at one part, and microcement on the other part. Both of these materials require the subfloor to be dry. I’m concerned this is going to take ages, as parts of this step are 19.5cm deep of solid concrete.

 

 

Use a decoupling membrane and you can tile it in less than a month from laying it. Saturate it with a 50/50 water /  SBR mix and allow to dry before sticking the membrane on, and use a good quality flexible tile adhesive and grout to finish.

Concrete continues to absorb ( consume ) moisture whilst it cures, so if this is internal then I would carry on as above.