Is it necessary to examine one’s conscience for exactly one hour before going to confession?
No. You take what time you need. Ideally it is best to examine your conscience before you go to bed each day. I would recommend making a list of the sins you committed throughout the day in a spiritual journal, and praying an act of contrition before going to bed. That will at the very least cover the venial sins. Even so, you should go to confession at least once a month for the additional grace regardless of whether you’ve committed grave sins or not. Your list will be ready for the time of confession.
You’d be amazed at how many venial sins we totally forget about on a regular basis. I still haven’t gotten far enough to write a list before bed, but once I started going to confession frequently, I realized that going to confession infrequently means you have no real idea or comprehension about how much you are actually sinning. This leads to not seeing the effects of the grace of the sacrament, especially if you only go once or twice a year and that leads to complacency with the reception of the sacrament. You don’t remember most of your sins, you don’t see yourself grow in virtue and as such you don’t see how the sacrament has any positive effect on your life.
If you haven’t made a list each night, its good to have sufficient time before confession but there is no dead set amount of time that it takes. Sometimes though I refrain from going because I have no time to examine my conscience to make a good confession and I know I’m in the habit enough to go frequently enough that I know indeed I will go again because I’ve been living my life going to regular confession (mostly bi-weekly, but not more than a month inbetween) for a decade. However, if you’re struggling to get yourself to confession, I’d take what time you have available in the line and do what you can.
Its like the parable of the sower of the seeds. God pours grace on us in all the sacraments. Sometimes we’re the road, sometimes we’re rocky ground, sometimes we’re covered with weeds. These things can all weaken our receptivity to grace. A good examination of conscious with regular confession can boost your receptivity to grace and you will find the sacraments far more fruitful than if you merely go through the motions.
Hope that helps.