Concrete subfloor with 2x4 on top
if you are going to lay hardwood on to the subfloor, then i assume its going to be nailed down and not floating.  if this is the case, then the floor has to not only be flat, but also strongly attached to the concrete below (to prevent deflection over time from moisture changes in the oak and in the subfloor).  therefore, you would want the concrete to be hard, but not necessarily fully cured, before you start anchoring your 2x4's.  assuming you will use construction adhesive and concrete screws (tapcons or something similar), you will want a low moisture content as well.  since tapcons need a compressive strength of about 20MPa to work properly (a little less if its a smaller diameter), and since it takes about 7-10 days for new concrete to hit that as a minimum, i wouuldn't be anchoring anything until about 2 weeks later.  if you go to soon, your anchors will strip out more easily and your adhesive won't bond well due to the moisture in the slab.