The specific answers to these questions will vary with the individual learner because each person’s goals for what they want to achieve in the language will be different. However, thanks to Master Steve’s teachings, I do think two general “rules” apply to everyone. 1) No matter what your proficiency level is, no matter how good you are, you can always improve; and 2) No matter what your proficiency level is, no matter how good you are, you will always WANT to be better. In other words, you are never really “done,” learning languages, like any learning, is a lifelong process.
That being said, if you want to learn more than one foreign language, at some point the second foreign language is going to have to get priority attention and the language you were previously learning has to take a back seat. In the past Steve has suggested your time be split at least 80%-20%. In my case that would be 80% work on Russian or French and 20% on Spanish.
Now, more specifically to your first question of how do you know when it’s time for that first foreign language to take a back seat: I think it is when you have met your goals in that language. For me and my Spanish, that was, at a minimum, fluency. I wanted to be able to converse comfortably with my Spanish-speaking peers on a wide variety of personal and professional topics of interest to me. I also wanted to be able to understand a lecture on a professional topic of interest to me, watch movies and telenovelas, documentaries, talk shows, and the news (especially the Mexican weather girls). I wanted to be able to read the newspaper, novels, and nonfiction books on topics of interest. I can do all that now.
But remember, you can always get better, and want to be better, so I wanted to find some numerical, definable benchmark by which to measure myself, as well as to know when it was okay to say “I’m done.” For me that was the Advanced Level 3 All Time LingQ stat goals, and then some of my own: 33,200 Known Words; 45,000 LingQs made; 500 Hours of Listening; 16,000 words of Writing, 50 Hours of Speaking; and especially 2 million words of Reading.
Most importantly, I wanted to spend enough TIME with the language, so I counted every hour of my learning, eager to put in the 960-1,000 hours required to learn Spanish for an English speaker as measured by the Defense Language/Foreign Service Institute. (24-25 weeks, 40 hours of study). Right now I’m at 1,300-1,500 hours.
So, in summary, I’m there. The only real things left on my Spanish to do list are taking a solo trip to a Spanish speaking country (as a reward and vocabulary activation effort) and maybe passing a C level DELE exam. I’m not sure I’m going to bother with the latter, the diagnostic test, which I took “cold,” put me at Level C1.1-C1.2. Since I have a lot of other things going on right now, and have put all language activities on the back burner for the moment, I have some time to decide whether I want to pursue those last two items or move on.