How much was Plato using authorial license in his dialogues centered on Socrates? Did he merely refine already existing arguments of Socrates' or was Socrates mostly being used as Plato's mouthpiece here? A work like Republic is so meticulously thought out that it's not unfair to believe it wasn't just the product of some half-drunk conversation. Since Socrates was Plato's teacher, how much of what he actually said informs his character in Plato's dialogues, if at all?
The problem is one of sources. Since Socrates himself didn't write anything, how would we know where he ends and Plato begins? How can we tell what the actual Socrates thought, when nearly everything we know about him comes from two of his students, who each present a radically different picture of his character and interests? As far as we can say with certainty, there is no philosophy of Socrates. Plato and Xenophon both begin at the beginning and write about their own ideas.
The so-called Socratic Problem has plagued scholars for milennia. While it is reasonable to assume that Plato and Xenophon didn't invent details of the man's life, so that we actually know a fair amount about who he was and what he did, it is impossible to know which of the views they ascribe to him were actually his. Once he became known as the archetypal philosopher, putting ideas in his mouth instantly increased their gravity and apparent sophistication. There are very good reasons for both Plato and Xenophon to present their philosophies in the form of Socrates fanfiction.
We might hope that the survival of Aristophanes' comedy Clouds would provide answers, since it was written when Socrates was alive and specifically features him and his supposed teachings. However, as a source, this comedy has problems similar to Plato's and Xenophon's Socratic dialogues. We do not know if Aristophanes meant to mock Socrates in particular, or just sophists and their craft in general; we do not know if he was representing anythng accurately or if he was just creating a grossly distorted caricature. Again, we cannot find the Socrates in this. As far as modern scholars are concerned, your question cannot be answered.
This won't answer your question in the least, but 'The Hemlock Cup' by Bettany Hughes is pretty good: it gives a lot of background and context for Socrates' life and really drives home how long he lived and how his period of fame as a philosopher came relatively late in his life. 'Why Socrates Died' by Robin Waterfield is also a very good book, but deals more specifically with his trial and execution at around 70, many decades after the zenith of his philosophizing.
I think that it is pretty clear that Plato was using Socrates as a literary character (and mouthpiece for his own views) in most of his works - however, that is just an opinion with no evidence to back it up, although I have in fact read most of Plato and most of Xenophon, but that was 25 years ago.
This is a totally unoriginal thought, but there are interesting parallels between the transmission of Socrates as a personality, and also of the content of his thought, and that of Yeshua ben Yosef (considered as an historical figure). All we know of either of them is what some of their disciples (who had agendas of their own) wrote about them later.