For what it is worth I see the time involved as the following.
1) Basic ability to communicate, to speak and understand, with difficulty, dealing with a limited range of familiar subjects..
If this is the goal, it can be achieved in 1-3 months of full time study in an immersion setting, depending on the difficulty of the language. If ,as as my case, we study an hour or so a day, and the goal is first to understand the language, this stage can be delayed for 6 months to a year or more.
2) The ability to discuss and understand comfortably, dealing with most subjects. In other words fluency.
This stage can be reached with 6-18 months of full time study, and can take years of part time study, again depending on the language.
3) Mastery, good command of vocabulary, and few mistakes if any.
This stage requires years of using the language, and continued efforts to acquire vocabulary and to notice things in the language.
I think most subjects means most conversations amongst friends, and most subjects on TV, radio etc..We understand and can communicate, more or less regardless of the subject as long as it is not some esoteric specialist field.
There is a difference between fluency, my 2) above, which corresponds to B2 in the CEFR, in other words, mistakes and a somewhat limited vocabulary, forcing us to tailor what we say to the words that we can use, and true mastery as with the example of that fellow Vaughan who owned language schools in Spain.