You need to know why you’re learning something in order to know when you’re done. What level of perfection does it need to be learned to? Which details should be learned and which skipped? That depends on its purpose.
At first, you can go by intuition or conventional defaults for how well to learn something, but it’s important at some point to start getting some control over this and making it more intentional and chosen.
To get a grasp on the purpose of learning, you need a tree (or graph). Writing it down helps clarify it in your mind. If you think about it without writing it down, there’s still information in your head that is logically equivalent to a tree (or graph) structure. If you have a goal and something you plan to do that’s related to the goal, then that is a tree: the goal is the root node and the relevant action is a descendent.
A tree can indicate some things you’re hoping to build up to. E.g. the root node is “write well” and then “learn grammar” is one of descendants. But those aren’t specific. How will you know when you succeeded?
It’s OK to sketch out trees with blank parts. You have the root node, then don’t specify everything, and then you get to the grammar node. You don’t have to know exactly what’s in between to know there’s a connection there. Figuring it out is useful though. It’s better to have something pretty generic like “learn mechanics of writing” in between instead of leaving it blank.
If you want to be able to write an article sharing your ideas about dinosaurs so that three of your friends can understand it, that’s more specific. That clearer root node gives more meaning to the “learn grammar” node below it. You can learn just the grammar that’s relevant to the goal. It helps you know when to move on. For example, you can write understandably to your three friends without using any colons or semi-colons. But you will need to understand periods, and you’ll probably want to use a few commas and question marks. And you’ll need to understand what a sentence is – not in full detail but at least the basics.
Another descendent node is “learn vocabulary”. Since the goal relates to dinosaurs, you’ll need some uncommon words like "cretaceous”, but you won’t need to know “sporadically” or “perplexity” (which are sometimes called “SAT words” due to showing up on the SAT college-entrance test – if your goal were to get into more prestigious colleges than you need to learn differently vocabulary).
Bottlenecks and breakpoints are important too. Which areas actually deserve much of your attention? Which are important to your goal and should be focused on? Which aren’t? Why? Usually you can get most stuff to a “good enough” level with little attention and then focus most of your attention on a few areas that will make a big difference to the outcome. If you can’t do that – if there are a lot of hard parts – then the project as a whole is too advanced for you and therefore needs to be divided into more manageable sub-projects. The number of sub-projects you end up with gives you a decent indication of project difficulty. If you have to divide it up into 500 parts to get them into manageable chunks, then it’s a big, hard project overall! If it’s 3 chunks then it’s harder than the average project but not too bad.
A bottleneck is a limiting factor, aka a constraint. If you do better in that area, it translates to a better outcome on the final goal. Most things aren’t bottlenecks. E.g. consider a chain. If you reinforce most links, it won’t make the overall chain stronger, because they weren’t the weakest link anyway. Doing better in that area (that link is stronger) doesn’t translate to more success at the goal (chain holds more weight). But if you find the weakest link – the bottleneck – and reinforce that link, then you’ll actually have a positive impact on the goal.
A breakpoint is a significant, distinguishable improvement. It makes some kinda meaningful difference instead of just being 0.003% better (who cares?). For example, I want to buy something that costs $20. Then there’s a breakpoint at $20. If I have $19 or less, I can’t buy it. If I have $20 or more, I can buy it. The incremental change of gaining $1 from $19 to $20 crosses the breakpoint and makes a big difference (buy instead of can’t buy). But any other $1 doesn’t matter so much. If I go from $15 to $16 or $33 to $34 it doesn’t change the outcome. More resources is generally a good thing, and money is generic enough to use on some other project later, but it’s important to figure out what will make important differences and pursue that. If we optimize things that don’t matter much, we can spend our whole lives without achieving much. There are so many details that we could pay attention to that they could consume all our time if we let them.
More specific goals are easier to achieve. More organized approaches are easier to succeed with. Some amount of organized planning – like connecting something to a clearer goal or sub-goal – helps you figure out what’s important and what’s “good enough”.
If you want to learn much philosophy or be much of a general intellectual, you need to be a decent reader and decent writing so you communication to and from you can happen in writing. And you need some ability to organize ideas and organize your life/time. It doesn’t have to be perfect but it has to work OK. And you need some general competence at most of the common knowledge that most people in our society have. And you need some interest in understanding things and some curiosity. And you need some ability to judge stuff for yourself: Does this make sense to you? Are you satisfied? And you need some ability to change and to consider negative things without getting too emotional. Those things are general purpose enough that it doesn’t really matter what specific types of ideas interest you the most, e.g. epistemology, science or economics, they’re going to be useful regardless.