I heard 3-5 years before ANH. I think they said that they plan the series to segue directly into the beginning of the original film.

And let's be honest. The whole bit with Yoda and Ben as the last of the Jedi was pretty much always a load of bunk. Even in some of the earliest EU, we saw Jedi who survived the purge. That old, homeless woman from Dark Empire for instance.

and as for Yoda seemingly not noticing Ezra? You're talking about the dude who chatted it up with the Dark Lord himself yet couldn't sense him. Sorry, but Yoda IS fallible. Plus Ezra may not even become a proper Jedi. Kanan wasn't even a Jedi. He was a Padawan at best by the time of the purge. It'll be a noob training a noob.