Upvote:1
As explained in Do I need a visa to transit (or layover) in the Schengen area? Egyptian citizens do not need an airport transit visa anywhere in the Schengen area except in the Czech Republic. So if your route allows sterile transit, you would not need a visa.
I could not find any definite information about Athens but the airport does not close at night so overnight transit should be possible. While I am not able to cite any explicit rule, I think that 24 hours is usually the threshold, if only because it's often necessary to collect your luggage for longer transit, which also implies leaving the transit lounge of the airport.
If you need to leave the airport for any reason, you would need a visa.