That's a good point I hadn't thought of. Many fields probably won't be like what I described: one would need to know a lot both about the field and about the person who needs advice, to give personalized advice.
Still, I think in most fields good personalized advice requires many years of studying and working in the field, while a few weeks of studying the person who needs advice would be sufficient. There is a disparity, partially (wholly?) arising from the fact the expert is already experienced in the field when they start working with the client, and has also studied how to analyze clients' requirements.
Of course, like you say, professionals that most people are able to hire can only give them a few hours of their time at most; often much less, like the 10-30 minutes of a typical doctor's visit.
Lukeprog has done some great posts talking about how scholarship has benefited him personally and how to do it. However, he hasn't talked much about how to tell the good from the bad, and I think this is an especially big problem with it comes to "how to"s, rather than standard academic subjects.
The thing is this: writing on academic subjects tends to be dominated by academics seeking to gain status from their work. Not all fields have a strong correlation between status and making actual discoveries, but many (perhaps most) do. So for academic subjects, the big challenge when it comes to making sure you're not reading nonsense is to try to evaluate the soundness of the field in question.
When it comes to seeking out "how to" advice, though, writing on the subjects seems to be dominated by people looking to make money selling advice. That means that if bad advice will sell better than good advice, they'll give the bad advice. Even if their writing includes legitimate advice, they have incentives to give a distorted picture of the subject they're writing on if it will sell better (for example, making whatever they're talking about sound easier than it really is). In some cases, advice books end up trying to nudge you towards buying some more expensive thing the author is selling.
So: when you're researching a "how to" topic, how do you tell the good advice from the bad advice?