There’s not a strict timeline. I wouldn’t be holding onto an idea that it should be done in X number of weeks in flower. While 12 sounds long, I’ve heard some strains take that long. They’re individual plants and each can take it’s own time. Check daily or every other day with the loupe and cut when they’re ideal.
Also, no dis on your friend but how long someone has been doing something isn’t necessarily an indication of doing something correctly or being an expert. Coming from someone with several other hobbies, as this is true in any hobby, it’s the old timers especially that can be stubbornly set in their ways with outdated information, bad habits, and old wives tales. Your best bet is always to take in info from several reliable sources and compare what each said, do your own research, then decide for yourself.