I have booked a flight from IST to ATH in J (award) and it's part of a larger itinerary (xxx-IST-ATH-xxx)

I'm thinking of booking an economy return (IST-ASR or NAV, depending on the flight time). When I check in at ASR, can the checked bag be sent all the way to ATH? Both flights will be on the same day. Or should I be prepared to collect the bag in IST?

Also, how much time should I expect to stay in IST for the domestic-international connection? I read that the airport is massive. The gap between the two flights (ASR-IST and IST-ATH) is either 2.5 hrs (seems short) or 6.5 hrs (seems way too long)...