It depends on the size and what you are trying to do. For example, lately I 've spent maybe a week studying Lucene, and kept going back to it every other day,

It depends on the size and what you are trying to do. For example, lately I 've spent maybe a week studying Lucene, and kept going back to it every other day, because I needed to understand pretty much everything about it, to get ideas for improving Trinity ( https://github.com/phaistos-networks/Trinity https://github.com/phaistos-networks/Trinity ). Some other codebases are so vast it takes a lot, lot longer to understand them enough to feel 'comfortable' navigating them (e.g the Unreal Engine codebase). Most codebases however are quite small, and within 1 hour, or a few, you 'll be able to understand where to go to find what you need, which are the primary data structures and functions, etc.