It's one of those things you can do nothing about, but, if you want to, you should be able to tell what the situation is by keeping an eye on
https://www.atoz-guides.com/coronavirus-in-greece-update/
Population of Greece is about 10.5 million, so for them to breach the point were you might have to quarantine, I reckon they'd need to be hitting between 2,100 and 2,200 cases a week. At the moment, they're getting sort of 1600 to 1700 a week, but it's rising.
If it starts being consistently over 350 new cases a day, then I think you'll likely be quarantining. If you're going for a week, you might get away with it, but it's a real knife edge situation. Most of the cases are round Thessaloniki and in the Attica region ( so round Athena ) and the hope probably is that as mainland Greece gets back to work in September, then cases will start to drop, but that's hope and conjecture really.
Despite that, I'm well envious of you, I've been in Greece at some time each year for the last twenty years, mostly pleasure, but sometimes working, and it looks like I ain't going to make it this year.