I think it varies based on the style of the presenter and whether they like to keep it loose or prepare every word.

I like to prepare. My best speeches were the ones where I wrote small ideas I could potentially say on post-it notes, and stuck them to a large wall.

Then, I re-arranged, edited, added, deleted, etc. until I liked the story line.

Then, (a la Tim Ferris), I break it up. I practice each 10 minute section separately, being able to deliver each 10 minute section 10 times in a row.

And just remember: presentations are not 'What do I say next?' but 'What do they need to learn next?'