I went to confession before entering the Church at the Easter Vigil. My previous baptism (as a child) was judged to probably be invalid, so I received conditional baptism at the Vigil.
You can imagine that after 40 years or so I had a rather extensive list of things to confess. In preparation I made the most thorough examination of conscience I was able, and confessed everything I was able to remember. Quite a few were along the lines of “I did Humiliating/DisgustingMortalSinXXX more times than I can possibly count, starting when I was YY years old”. (Not that some mortal sins are less bad than others, mind you) The whole thing took maybe 10 or 15 minutes, I don’t really know.
What’s required is to confess the sins you are able to remember, and have the willingness to have confessed the other ones if you could have recalled them.
There’s a really good book entitled Frequent Confession - Its Role in the Spiritual Life, by Benedict Baur.I highly recommend it.
Confession is AWESOME. You’ll be very, very glad you did it.