Overthinking and dwelling on things is a real problem, particularly for people with anxiety disorders. And the suggestion that you can’t think about something for too long is ridiculous.
Clearly a disorder is something else other than healthy deep thought. Overthinking, dwelling (and brooding as the other commenter said) isn't the same as contemplative or investigative thought about a topic. Rather, it's the opposite and isn't very thoughtful at all.
Too long is so long that time spent thinking on the thing disrupted or took away time from some other higher priority thing / task / whatever. Or so long that it delayed the decision / outcome / whatever enough that it had materially negative impact compared to making a suboptimal, but earlier decision.
