If you have FileMaker 13:
Use the Oval tool to put an orange circle on your layout while your layout is in Layout Mode.
With that oval still selected enter this expression into the "hide object when" box on the Inspector's data tab:
Get ( CurrentTime ) < ( YourTable::StartTime + YourTable::Hours * 3600 ) // multiply by 3600 to convert hours into seconds
You can include more code to hide it when your starttime and hours table are empty as well:
IsEmpty ( YourTable::StartTime ) or Isempty ( YourTable::Hours ) or //put above calculation here.
There are ways to do this with older versions but they are a bit more complex to set up.