Living is all about memory. Honestly. Everything you are (to yourself) is your memory. There'd be no difference if you just came into being right here right now, but with all the memories you have being artificially implanted (think about The Island). Skills and knowledge are all part of memory. So, if you're picking up a new skill, what's the best way to do it?
I've long been frustrated, and I'm sure many of us have all been at various stages on the learning curve, when picking up skills in the sense that we would like to be better than what we are, and hence continue striving forward. Thing is, it kinda seems like, there's an optimum amount of practice and work which helps a skill (or knowledge) ingrain and implant itself in us. I've experienced this with piano, Jubeat, Reflec Beat, Sound Voltex, badminton, bowling, and I can't remember what else. Practising too much can in fact, destroy skills and cause bad habits that are hard to get rid off in the future (some of these still plague me). Similarly, with say, building up on knowledge, memorising something too often and frequently might sometimes cause mis-association and confusion, although, this is no doubt, less likely to happen unless memorised without understanding (which, for example, when learning a language, is inevitable since there's nothing to "understand" about vocabulary), and comes with a higher risk if particular mnemonics are used.
I've also found that when practised "into a rut" that I can't get out of, leaving it for many days (e.g. 3 weeks to like 2 months) and then returning and practising in earnest, can clear the rut and often result in superior results overall. So, the question is, how do you prevent getting into a rut, and more importantly, what's the optimum amount of practice one should do to achieve the fastest and most efficient results possible?
Turns out, it's as long as your concentration can hold, AND, not more than 4 hours in total a day. In fact, the max efficiency stops at around 2 hours. This is for musicians, but I think it applies more or less to every other skill. Don't count warm-ups of course, since unless they're drills or something, your "active learning" isn't there. Speaking of which, it has to be "deliberate practice" or not much will come of it, and might hurt you on the contrary (the rut thing).
Memory requires a slightly different treatment. To memorise something, you periodically need to refresh (much like DRAM), but each time you do, it sinks deeper into your mind, and hence the next refresh will be longer. This is called "spaced repetition", and is very effective in helping retention. Unfortunately, the intervals are not specific, and not that well documented. A pretty successful algorithm is the SM2 which was used in Supermemo, but experiment with this I'd say. Personally, I think the spacings (for me at least) work best at: (below one hour, almost incessant), 1hr, 6hrs, 1day, 3days, 7days, 7days (again), 2 weeks, and then I don't know...
I find Anki pretty good with the initial spacing in its algorithm, but subsequently, the spacings become too far apart, and I quite easily lose them, so once again, it varies from person to person.
Meanwhile, why do I need to consider all this? I have 5 more weeks to learn German.