My condensing fan motor is humming. I just replaced the capicitor 2 weeks ago and visually it looks OK. Will the motor hum if the unit is in defrost cycle?

No. The condenser fan motor will be doing nothing when in the defrost mode... Why did you replace the capacitor?

The fan motor was bad so I replaced it and the cap as well. All has worked fin until tonight. If in the defrost mode would have it been the compressor I heard? How long does an average defrost cycle last?

Yes, you would hear the compressor. It will last only a few minutes...