Kevin Kelly says that 7 'why's are enough. As in, if you ask 'why?' 7 times, that's about as deep as you'll need to ever go.

I've heard that if you read the top 3 books/articles in the subject, that's pretty much enough too.

Pareto principle works too. Aim to learn the 20% of concepts that will give you 80% of the material

The Feynman technique is also great. You try to teach the material back to someone. Chatbots are great for this one if you can manage the prompt well enough.

Decide in advance how much time the topic deserves. No cluse as to how to do this well though, sorry.

Ask yourself: 'will knowing more about this change my decisions?'

In fast-changing fields, it’s more useful to set a rhythm for refreshing your knowledge (quarterly or annually) than to go endlessly deep once.