The lowest time you can get it down to is seven minutes, as long as you and Pinkie work together.
Arbanis describes the process here:
"Minute 1:
You prep frosting 1, Pinkie preps frosting 2
Minute 2:
You prep frosting 3, Pinkie frosts cake 2 (1/3)
Minute 3:
You frost cake 1 (1/3), Pinkie frosts cake 2 (2/3)
Minute 4:
You frost cake 1 (2/3), Pinkie frosts cake 2 (3/3)
Minute 5:
You frost cake 1 (3/3), Pinkie frosts cake 3 (1/3)
Minute 6:
Pinkie frosts cake 3 (2/3)
Minute 7:
Pinkie frosts cake 3 (3/3)
Minute 8:
Close up!"