That would be 27-37 years, actually.
My understanding is that there's a good deal of evidence that Matthew and Luke both draw upon the gospel of Mark in their writing. There's also a hypothetical document or set of quotations or oral tradition or something called the Q Document that would explain some observations about similarities between Matthew and Luke or somesuch. I don't remember too much about the Q Document stuff, though; it was mentioned briefly when we discussed Luke in relation to Acts in a class I was taking. I do, however, remember that Luke is /explicitly/ a secondary source by self-admission... see, well, the first four verses. It does claim to be based on eyewitness accounts passed through oral tradition, and oral traditions would be another... well, okay, not "document", but source to pull from. Part of said traditions could be the Q document, I think, actually... I think I'm rambling a bit. Anyway, look up the history of Matthew and Luke, I guess, and see if you find something like what I said. I know basically nothing on John, so I won't touch on it.
A lot of history /is/ questionable. Because, well... reconstructing the past is /really freakin' hard/. I don't think that, in the specific case of saying that the Gospels themselves support the existence of a historical Jesus, there's enough evidence to consider them a solid grounding for the hypothesis by themselves. Also note that I'm just arguing the Gospels, because it's the only part that I have much familiarity with at all, though; it is possible that there's a confluence of other evidence I don't know about that could support a historical Jesus.