There's an interesting write up on how much heat gets dissipated...
2A load --> 2.0W
6A load --> 9.6W
http://forums.parallax.com/showthread.php/71139-Darlington-Transistor-vs.-MOSFET
Might need a heatsink, or a revised circuit using a logic level mosfet.
EDIT (more test data):
Collector voltage (drop from collector to emitter) = .774 V
Voltage across load resistor = 13.0 V
Current though load and transistor = 1.33 A
Power delivered to load resistor = 17.3 W
Power turned to heat in the transistor = 1.03 W
TIP120 Temperature = 191 deg F
Temperature rise over ambient = 120 deg F