I self taught myself Japanese for many years after attending 6 lessons.
I'd say I'm ok at reading Japanese for a self-taught person. My Japanese comprehension and vocabulary is around N3 level.
But the tricky part is, whilst it is easy to learn reading and writing, it's difficult to self learn pronunciation and listening.
Or for some people, it may be the opposite.
Basically, it's difficult to control whether you are learning properly in all disciplines (reading, writing, speaking and listening) of the language.
My listening in Japanese is terrible. So terrible, that I can't get 100% in N5 level listening, which is awful.
Moreover, it has been over 10 years by now, but I have still not reached business level in Japanese in any discipline.
That's dreadful because averagely speaking, an average Hongkonger who attend proper Japanese lessons, should be able to reach business level Japanese within 3 years.
So, in other words, if that language is important to you, you better attend proper lessons.
Only try self teaching if it's not important and that you want it as a hobby.
It's funny, your description of your Japanese ability sounds just like my Mandarin one. It's amazing how much you can comprehend just by knowing the characters. Japan and China are the two biggest powerhouses in my hobby, so after a huge international convention recently, I made a whole bunch of Chinese friends. I'm sure that if we were trying to talk in person it would be terrible, but since it's mostly messaging over Facebook (quite a few VPNers) or WeChat, it actually works out with a bit of help from bing/google translate.
3 years does sound about right for a native Chinese speaker to reach Japanese N2 if they dedicate real time to study (like 1-2 hours a day every day), in a classroom setting. 2 Years probably close to doable in a real intensive program that also has private tutelage, 1 year if we going full on military-resource-immersion-program.
I have an N1, got it after studying Japanese for... 7 years I think? It's already been 11 years though... Wow, can't believe it's aready been 4 years since I got that thing... but I feel my business Japanese has only gotten really good in the last 2, living in and working in Tokyo.
There were definitely time periods when I was self-teaching myself (maybe about half that time), but I also had classroom experience and brief full-blown-intensive-immersion programs (3-5 hours a day).
Also on or off, I had the fortune of loving, and having access to Japanese media. Actually, it was one of my self-taught periods that really pushed me from a really casual learner (maybe about N4-ish) to a high-potential learner (speaking level high enough to reach N2 level overall abilities with a bit of dedicated training). Basically, I watched Lovely Complex 40 times. Yes, I watched all 24 episodes about 40 times each during one semester of College, and that basically took me over a massive breaking point in the learning curve. My interest in being able to comprehend Kansai-dialect forced me to genuinely learn good grammar structure, and honing in on the "music" of Kansai-dialect massively improved my overall intuition for the language.
Also, there are simply things that you cannot hear or understand on the 2nd or 3rd time watching something that you can on the 10th, 22nd, 34th, 50th time hearing it. There are a lot of people who like to "learn from anime", but very few enjoy anything enough to watch it so many times; which is a huge tool for me.
When I watched Ryoma-den for instance, I was already far above N2 level but I couldn't comprehend a lot of the grammar being spoken by the characters in the many different old dialects that appear in the drama. But by the 10th time watching every episode of the first 20 episodes, I was basically able to understand what all of them were saying in Tosa dialect, Choshuu dialect, Satsuma dialect (especially annoying), etc.
Media is huge imo.
The lack of Chinese media that generally interests me is probably the biggest wall to me improving there. That and being married and not having nearly as much free time for language anymore. lol