I guess it depends on what you're dealing with.
I've had a situation going on for many years where my partner has had a lot of anxiety and worry.
For years I tried to intervene or help by dealing with whatever the issue of the day or time might have been. Over a longer period I started to suspect that my partner's worry was out of proportion to the challenges she was facing, moving from vehicle to vehicle, as it were, to provide a reason or a focus to worry. But by that time I had become so used to her anxious behaviour that I think I had in some sense become an enabler of that anxiety by trying to work around it rather than confront it.
Further years down the track and my partner is in therapy, seeing some great results, and now accepting a diagnosis of general anxiety disorder. It seems to align with her sleeplessness and constant worry. I'm not entirely sure how the diagnosis helps her beyond her therapy, but it kind of helps me oddly enough. It helps me feel like I don't have to take it all on.
So for me the question was not so much when to intervene by itself, but when and how. The first thing in so many respects was trying to get a handle on what we were dealing with. I would wish for others in a situation like ours that a person could somehow know what was "normal" and what not in terms of worry. Seems a big ask?