How long is too long when writing an answer?
I found that writing around 10-15 sentences seem to be enough to answer a question.
More than that means that I haven't really looked carefully at what the question is asking.
The question should be answered first and then more should be added to that. If you add too much, the person asking will have trouble understanding it.
Put yourself in their shoes and see if your answer makes sense.
I find that images and drawings are useful. There are also many videos that help a lot.
I am a visual learner and those images and videos work well for me.
Ideally I think answers should teach rather than simply solve a problem. So what do we have? Why do students come to Socratic?
Some possible reasons:
They are genuinely stuck on a question due to not having understood something that they were supposedly taught.
They are too lazy to think about the question themselves and just want to get a solution with minimal effort on their part.
They are genuinely interested in the subject and have their own questions that are puzzling them.
They feel that they do not have time to solve the problem themselves.
They do not have sufficient help from their teachers and/or peers to press into new concepts.
Some things to consider when answering:
It's ok to spoon feed a slick answer, which should at least give a good example, but may be better to provide at least one concept to learn in the process.
The answer is not just read by the questioner. Ultimately, hundreds or thousands of students - with different levels of understanding - may see your answer.
It is often appropriate to keep the answer to the level of the topic under which the question is posted, but there is also the possibility of inspiring students with what lies beyond. Perhaps separate off such remarks under a "Footnote" heading.
Sometimes the student has been told to use a particular method when solving a problem (e.g. quadratic formula or L'Hôpital's rule). Your answer should certainly use the method which was asked for (or explain why it's not a good idea), but it may be worth spending a little time presenting some background on the method itself. For example, the quadratic formula is fairly quick to derive from completing the square, so you could briefly explain that - or link to another answer that does.
So how long should an answer be?
If you can teach something worthwhile that you think will have good impact then I would suggest that you go roughly by the warning messages that Socratic gives you. Like about this length.