July and august are very busy times, bu otherwise, 2 hours is perfectly adequate ... If you are flying out to Santoirni on Aegean or Olympic, it's all in the same building-- international one end, domestic on the other. By the time u get thru passport control (ther is NO customs line), your bags will be on the carousel, you can pick up some €€ at an ATM, and stroll to the domestic check-in. You'll have time for coffee.
JoeF may be speaking from his experiences in Italy or France, where capital cit y airports can be huge and sprawlling, Rome. Paris, (particularly the nightmare that is Chas DeGaulle) -- i don't see from his posting record that he's experienced Athens airport yet. I come in regularly on that 9:15 AA flight, and it's before the other 2 international flights (Delta & CA), so passport-control lines move quickly.
BTW, for future reference -- why why do people not say WHEN their trip will be, when time of year can make big difference... please remember for future, wherever you may be going. It helps forum contributors be more useful & not have to guess.