I think that for questions that are about rules, where there is a single, objectively correct answer the user can accept it if it is detailed enough for them. This is because the only alternate answers are going to be either repetitions of the existing answer, or wrong answers.
However, more experienced users will still tend to wait so that they have the best possible selection. This is good, but not always necessary. Some questions have the potential to be answered quickly and accurately, but could also receive a more in-depth answer with suggestions on how to avoid a problem, alternative courses of action, or more rules quotes/links.
For more subjective questions I think it should be discouraged to accept an answer for at least 24 hours, to allow all time zones to attempt a response. I think that @waxeagle's method of simply commenting to let the user know is effective - I have seen it used successfully many times.
Overall, we should let new users accept answers for simple questions without waiting long, as they are likely to want a quick and simple solution. More experienced askers, or longer questions, should be encouraged to wait by simply leaving a message for them.