It takes me hours to set up my alt’s WAs, binds, and UI, usually I do this while and after learning the rotation from IcyVeins and practicing on target dummies. It’s also a few more hours to learn the dungeons but this is usually once per role, and less to learn the second and third role. Learning about the customization options for the addons I like and how to use WAs could easily take 10 hours, maybe more, before you’re using them like a pro. Learning how to use macros and making them for your spec is probably an hour or two. Reading through talents and modifying them to fit your preferences might take an hour or 30 minutes if you use MurlokIO/warcraftlogs to immediately rule out ones that nobody takes.
I think if a player spends about 30 hours preparing, and then about 10 hours in future seasons, they will be great for M+.
It sounds like a lot and maybe it is, but if you enjoy M+ it could easily be 200+ hours of entertainment per year, so it might be worth investing time into learning it.