TBH it's very difficult to gauge when to stop pursuing something. Personally, I learn something up until it stops becoming useful and/or it starts becoming more of a burden on my mental overhead. Doesn't mean I'm never going to go back to it, it just means I accomplished what I needed to do with the minimum amount of knowledge. From there on out it is entirely dependent on how much the topic interested me. Most of the time I'll end up going back to a topic anyways since making things isn't just about designing and creating the thing, it's also maintaining it. At that point it's more about whether or not you can pursue a project due to it's knowledge requirements, but I believe that falls out of the bounds of your question.

This is my rule of thumb:

Learn what you need to know first for your task at hand. If your curiosity can't be contained, make a mental note. If that mental note doesn't go anywhere and you keep dwelling on the thought of it, pursue it until your curiosity is satisfied. If done successfully, repeat.

You COULD write it down, but if you're anything like me you'll just end up with a desk filled to the brim with post it notes and never actually going back to 90% of those topics. Which kills a lot of trees. If you're into that sorta thing, don't let me stop you. Making a mental note is much better since if the topic is of interest to you, then you will not forget about it. Therefore, you'll let your curiosity build up until you cannot contain yourself and have to learn more about the topic.

NOTE: Being a generalist usually comes because of a deep sense of curiosity in a wide range of topics. Take your time and enjoy the process of learning things since that will guarantee the longevity of your curiosity without burnout.