To Be Clear installing GM9 takes 20 minutes on average, for a first time user, it may take more or less time depending on the manual either github, reddit, or the official modding website. That is just the bare minimum of modding, luma, twilight-menu, and pretendo.

Take way more time than what @Cheese is referring to. Not including Flashcarts, and Emulators. It would take 30 minutes for a basic full client mod (GM9, Luma, And Pretendo) To be installed on a stock 3DS. All in all it would take 2-3 hours to get an advanced full client mod.

GM9, Luma, Pretendo, CTGP-7, 3DS backup+Flash (for the Japanese 3DS's) would take up to 5 hours or more. CTGP-7 is optional, and the time it would take to mod doesn't include even putting the games on your 3DS. But a basic mod (GM9, Luma) would take around 10 minutes.

Besides all that GM9 is the first thing you want to get on your 3DS.