I've read like... 100 pages of the book. How much more should I read before watching Dune (2021), so that the movie does not spoil anything from the book for me?
There's a page-break just after the paragraphs below. A guy called Jamis has just been killed and Paul and his mother are heading off to a sietch with Stilgar and his band.
Stilgar held up a hand, and there was silence. “As to Jamis,” he said, “I order the full ceremony. Jamis was our companion and brother of the Ichwan Bedwine. There shall be no turning away without the respect due one who proved our fortune by his tahaddi-challenge. I invoke the rite . . . at sunset when the dark shall cover him.”
Paul, hearing these words, realized that he had plunged once more into the abyss . . . blind time. There was no past occupying the future in his mind . . . except . . . except . . . he could still sense the green and black Atreides banner waving . . . somewhere ahead . . . still see the jihad’s bloody swords and fanatic legions.
It will not be, he told himself. I cannot let it be.
= = = = = =
That's pretty much where the film ends and most everything beyond this point is going to appear in the sequel/s.
I wanted to give an answer that risks no spoilers...
It's very hard to say where in the book the 2021 movie ends without spoilers, but about half seems right. There's a point in the book where about 3 years are skipped over. That's about where the movie ends. Edging close to spoilers, some significant conflict between the Atredies and Harkkonens occurs just before this break in story timeline. It's really no spoiler that there will be this conflict eventually. Numerous pages are spent discussing its inevitability.
Dune is actually already divided into 3 "books". By that measure, the movie ends just a small way through the second book, "Muad'dib". For contemporary Hollywood tastes, the demarcation between book 1 and 2 is far too nuanced to end a script. It signals a significant change in Paul of which the gravity would be hard to capture on film. But, there is a scene just into the second book that does give a decent ending for a Hollywood script, and that is where it ends.