The example with the for-loop vs. map/filter in particular - it's such a micro-function that whichever the original author chose is probably fine. (And I would be suspicious of a developer who claimed that one is 'objectively' better than the other in a codebase that doesn't have an established style one way or the other).
Refactor when you need to when adding new features if you can reuse other work, and when doing so try to make minimal changes! Otherwise it kind of seems more like a matter of your taste at the time.
There's a limit of course, but it's usually when it's extremely obvious - e.g. looong functions and functions with too many parameters are obvious candidates. Even then I'd say only touch it when you're adding new features - it should have been caught in code review in the first place, and proactively refactoring seems like a potential waste of time if the code isn't touched again.
The (over) consolidation of duplicated code example was probably the most appealing refactor for me.