What is the total running time of Database up-gradation from 10.2.0.3 to 10.2.0.4 in test instance?
under 1 hour
Why don't you duplicate a test instance and try it out for yourself, following the instructions in the readme file?
FWIW I would recommend upgrading to 10.2.0.5 if you are going to stay on 10g.
3# is right