It's not just time accumulated or words read or words learned
llearner

Another good metric is the ratio between Known Words and the total number of Lingqs within a specific time frame. It's a good indicator of efficiency in acquiring new vocabulary over time. Concentrating on more than one language at a time has been overwhelming for me. I would go at full throttle at a new language and idealistically get to the milestone such as watch first TV series, in which active maintenance is no longer required IMP. We need to spend more time at the beginner's level, and I devote more than two hours daily to learning to keep up with the momentum and the noticeable progress.
bamboozled

What is the bare minimum to maintain a language?
I think the amount of work necessary to maintain a language depends first of all on your level. If you're advanced less time is needed, but a beginner might quickly forget if they take some time off.
Therefore I hypothesize that there is a minimum amount of time needed
That sounds sensible. For Chinese I have found 3 hours per day on average to be a sustainable number while making some progress. Although I have to say, on a day to day basis, any progress remained below the noticeability threshold. Intervals at which I noticed actual progress would be more like 500 hours. So at 1000 hours I started to understand fragments of speech on Youtube, after 1500 hours I started to get the gist of videos from familiar creators (without subtitles).
I'm referring to hours of listening here as that's the simplest measure I have found, it works for me because I typically listen and read simultaneously. Juggling with stopwatches gets annoying really quickly, so I track pure reading only sporadically.
xxdb

If you're advanced less time is needed, but a beginner might quickly forget if they take some time off.
I've read in a couple of places that once you're at an advanced level the language is "yours" as in, it doesn't degrade very much. That seems to be true for me with Spanish. I hardly practice it at all and it's just there, ready for me, when I need to use it. I've forgotten some active vocabulary over the years but I never degrade to the point I was before I hit threshold comprehension - as in I understood everything.
For Chinese I have found 3 hours per day on average to be a sustainable number while making some progress.
In my own experience in the last couple of years when I started out in my initial six month grinds with French and now Russian I was doing between 3-4 hours a day and I made pretty good progress. I likely did 3-4 hours a day in Spanish for a year and a half to two years straight. With Russian, I've probably only done an hour at the most per day the last year with some bursts of a couple hours whereas the previous year I did 3-4 hours the first six months and maybe 2-3 hours the second six months. I suspect that now, however, I'm increasing in slight bursts and then retreating and forgetting some. One step forward somethings two steps back because while I am better than I was a year ago, the difference is not as dramatic as it was starting from scratch. So I suspect at least for me, the threshold where improvement is made might be at least two hours a day whereas one hour a day is a bare minimum not to suffer attrition.
I guess the answer is to grind it out for another year till I hit advanced.
jahufford

I agree that the more you have, the slower you forget... But I did 400 hours of Chinese in 1 year and felt I was making steady progress the entire time (with a LONG ways left to go, no doubt), maybe we just have different definitions of progress, because 3 hours a day to me would be a lot.. If I only I could discipline myself to get that much Chinese a day, ooooh the progress I could make, I get shivers just thinking about it... I've taken a few month break and feel I've forgotten a LOT, I hope it comes back quickly(had to refresh/improve my Spanish for an upcoming trip)...
Like you in Spanish, I can just come back to French and listen seemingly like I never left, although I seem to need a few minutes of warm up to get back to speaking decently, *especially* if I've been focusing on Spanish, Spanish keeps wanting to rear its head. A sentence will start in French but end up with pourquoi turning into porque.
bamboozled

The sense of progress is of course completely subjective. But what I'm referring to is really listening comprehension, progress with reading was much smoother. So for most of last year I was in a position where I could read along podcast transcripts while listening to them without problems, but not understand anything when taking the transcript away - a somewhat frustrating situation. This is where I found the statistics to be helpful, setting arbitrary goals in 100 hour intervals induces a sense of progress even if none can be seen. Of course many people feel this is too simplistic or are more results oriented, but since we're dealing with psychological phenomena anyways, I don't think it matters how one generates this sense of progress. Working towards milestones makes it also easier to keep up with the 3h per day goal, they make it obvious when I'm slacking off, this tends to be more motivating than frustrating.
jahufford

Dang, I just realized it was you that did 3 hours/day in Chinese and xxdb is the person with Spanish.. I conflated the two of you. My bad lol, sorry.
It's interesting to me this idea of a "sense of progress", as it ties into motivation and what's optimal.. Who cares what's technically "optimal" if you won't do it cause it's not enjoyable. If you sense you're progressing then you're likely to be more motivated to continue.. I also like milestones/goals to work towards. The numbers that lingq gives you are pretty cool.
Listening can be hard for adults. Seems like many/most of us are more visually dominated compared with children. I'm not sure why that is.
Dreaming_Butterfly

We can find the average, but in the end it will depend on the memory capacity of each individual and it is up to every person to find it by themselves. I imagine that the great difference between people that can learn more than 10 langages and normal people is the memory capacity. Some freaks may be able to check the meaning of a word and never forget it, almost every time. Did somebody tested the memory capacity of the superpolyglots? This is a nice exemple (seems to be a real story): https://www.youtube.com/watch?v=PPySn3slfXI&ab_channel=RealStories