Hi,
Assuming we are talking about paved road riding, the answer is: you can totally forget the rear break and use only the front break for emergency breaking.
The reason is that while hard breaking, the motorcycle weight is leaning most on the front wheel, so this wheel can provide much more breaking power. The rear wheel will almost certainly lock under hard breaking, and although locking the rear wheel is not a big deal for an experienced rider (locking the front wheel is always a big deal since you lose steering ability and bound to fall unless you release the break) the best strategy for emergency breaking is concentrate on the front wheel only and learn how to get most of it without locking it.
You should practice emergency breaking, using the front wheel, in a place free of traffic, (a parking lot, etc.) and practice locking the front wheel, releasing before anything gets out of control and breaking again. Only when you can do this almost without thinking, you can try using both breaks.
I think that tests show that an experienced rider can cut his breaking distance by only about 10% by using both breaks (i.e. if he can stop from a certain speed in 25m using the front wheel only, he will stop in 22.5m if using both breaks) while a beginner might even extend his breaking distance due to the need to take care of the locked rear wheel.