Keep supplying sugar-syrup until both brood chambers are built out and
occupied.
 
On some days the bees will tend to ignore the syrup, if there is a strong
nectar flow, but there can be rainy or cool days when nothing is available
in the field.
 
Also they can take in the syrup at night.  This constant supply of food
will ensure the most rapid construction of combs, and fastest colony growth.
 
Stop feeding when you give them the first honey super. By then the colony
is strong enough to be self-sufficient.
 
Rule of thumb, anyway.