There's a blurring between legend into pseudo-"history", and pseudo-history into real history. The timeline is something like:

• Trojan War (the last big pan-Hellenic cycle)
• the returns, exiles, and deaths of Trojan War heroes and their epigoni
• Dorian invasion of the Peloponnese led by the Herakleids
• Ionian migration to Asia Minor led by the Neleids
• miscellaneous foundation legends (Chios, Taras, Cyrene, etc.)
• mythologised 6th century BCE history (mostly revolving around disputes between Argos and Sparta)

So there isn't one single point at which the stories stop being myths and start being history. My own pick would be the foundation legends phase, but that's purely personal: there's nothing unique about that phase that makes it an interface between one thing and another. In that phase there are still heroes swanning about who had hero-cults in historical times; but really the same is true of the last phase too, albeit to a lesser degree.

I've written a couple of posts on the "timeline" of Greek myth before which might give a little more context: this one, and this one. If you have any follow-up questions I'd be happy to have a go at them.

As rosemary85 says, the mythologization of history continues well into the 6th century. The story I like to point people towards it Croesus, by Bacchylides, by Herodotus, by Xenophon, by Ctesius. They all four wrote for different purposes and took a different tact to the supernatural elements, with Bacchylides and Herodotus pretty gung-ho for divine intervention and the others less so.

It could be argued that Christianity is the last myth of the Greeks, but that might be too controversial and tentative a thesis to maintain.