I would say the biggest determinant of your speed is simply how much time you plan to devote, your dedication, and ability to stay on top of your lessons.
Generally, I think the semester system (which many of these textbooks were designed to fit nicely inside), is a good way to generally visualize how long each ‘section’ of study should take you, with each 6 months roughly corresponding to ~1 semester or so.
For example:
Genki I = 6 months (1 semester)
Genki II = 6 months (1 semester)
Tobira Part 1 = 6 months
Tobira Part 2 = 6 months
Time to JLPT N2 (lower Advanced) = 2 years
I passed N2 in about 2.5 years w/ self studying – it is totally doable, as long as you move at a swift pace. This is just a general guide, and does not need to be taken literally. I think the best way to pace yourself and make sure you are making good progress in your studies is to set a realistic pace and goal for yourself for chapters / Kanji Levels/Numbers per week, and then sticking to it. I think you can reasonably get started on the content of this guide / work in Japan in 2 years or less depending on your motivation and level of immersion. Certainly don’t be discouraged if it takes longer – its all about how quickly you want to move, and what your ultimate goal is for the language 🙂