I, too, heard the rule of "one hour for one minute". Must have been the thing in our time. I think it makes a point that adequate preparation is needed if the sermon is not going to sound 'off the cuff'. That said, depending on the week and the needs/size of the parish, finding those 15-20 hours might be a challenge, to say the least.
But it is true that sermon prep also takes place outside the study, as is noted. The sermon must be contextualized for a given congregation, so the needs and struggles of the people of that place must be in the forefront of the preacher's mind.
I also consult a variety of sources, sometimes including the sermons of others, such as Selected Sermons of Norman Nagel: From Valparaison to St. Louis (CPH 2004), mainly to see how they treated the text and what themes they may have discovered. Additionally I will consult my own sermons from three years prior when I last preached on the text. I do not always agree with how I did some things, or realize that circumstances have changed, and that launches me off into additional study to see if I possibly misunderstood parts of the text that I could understand better. For that good commentaries are a must.
This morning was an interesting day to revisit a prior sermon, as this Sunday, three years ago, was the first Sunday we were in 'lock down' mode in Wisconsin when our governor ordered public buildings to be at a 10 or less capacity. That Sunday was the first time the in-person 'congregation' consisted of only three people; a reality we lived with for two and a half months as I preached to a cell phone week after week. Historically I had approached the text of the healing of the blind beggar from John 9 in the context of the beginning of a pandemic with all of the fears and questions attendant to that event. Seeing a merciful God in the midst of suffering still seemed timely, and I revised that sermon with the added perspective of how God brought us through that difficult time.
I don't think AI could have done all that.