There are many activities in life where a certain level of effort is required and without that effort the final objective will never be achieved not matter what the time scale. I can decide to run for fitness, but if I wish to run a marathon in a respectable time then I have to invest the effort in training. Simply running for half an hour once a week may make me a little fitter but even after ten years of this I will not be ready for that marathon.
I believe that to learn a language you have to be prepared to work hard, either at a certain level all the time or occasionally in concerted bursts. If most people (there may be exceptional minds to which this does not apply) don't do this then they will only ever achieve a superficial grasp of the language, no matter what the timescale.
My understanding of the mechanics and chemistry of human memory is somewhat superficial (I haven't made that concerted effort to master this kind of knowledge). However there does seem to be an element of "use it or lose it". This use it or lose it principle would suggest that if you only learn in a casual manner then you will reach a point where you are just "treading water", a lot of your effort will be reviewing things you originally learned long ago or the new things you learn will be offset by the old things you are forgetting.