I've recently started using the pomodoro technique. As a result, if I can't answer a question without breaking my train of thought on my current task, I have started asking if I can postpone an answer until the end of the pomodoro, an average of around a 15 minute delay. An interesting side effect of this I have discovered is when I drop by their desk to answer the question, they have often already solved it on their own. If they haven't, at that point I am much more prepared to give them my full attention.
This isn't school. It's not cheating if you quickly provide a fact they could eventually find on their own. On the contrary, it makes good business sense to save them time, and in my experience skills are sharpened very little by trial and error compared to a mentor giving you frequent small pushes in the right direction. I'd rather them learn 10 right ways to do things with my help than 9 wrong ways and one right on their own.
If something can be easily looked up, teach them how to do so. On the other hand, if it's something you can only know from experience, like which files to investigate for certain bug symptoms, I see absolutely nothing wrong with just giving an unexplained answer.
Conversely, more subjective things like architecture guidance should always be accompanied by the reasoning behind it. For one thing, the junior developer has thought a lot more in depth about their specific task than you have. Talking it through makes sure you aren't jumping to conclusions. For another, it keeps them from blindly applying rules to future situations where they might not apply.
I can only think of one case where I outright refused to continue helping a coworker, and it was after spending a couple hours explaining something multiple times and going through several examples, after which she literally still didn't know the next statement to type with some very leading hints. At that point she had little hope of keeping her job without some serious relearning of fundamentals, and sure enough she only lasted a couple months.