Should I select an answer if it's the only one?

Question

Is there like some logical time limit where if no one else answers for x days/weeks/months than I should just give the best answer to the one answer, even if it's not particularly good?

Answer 1

I would say no. BA can be taken as an indication the answer solved your issue or as an endorsement of its correctness. If it's not a good answer, then it doesn't help other people to select it.

Comments

What if something's the first one? if it's useful, I would probably select it, but then if another, better one comes along, should I de-select it and select the other one? I don't want to be that guy that always takes other people's 20 points away.

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Yes, you should select the better one (it's called "best answer" after all).
You would do well to stop worrying so much about points and just do what is most helpful to people reading the question in future.