Magnitude of \$I_{CBO}\$ in a typical transistor
In a real application there is typically voltage between emitter and collector, so the \$I_{CBO}\$ current can effectively be base current and thus be multiplied by the gain and cause a lot of dissipation. 

"Is" in the SPICE model for a given part will give you a sort of "typical" value for that particular part at room temperature. It is usually in the nA range at room temperature.