I am not certain of what you mean by the expectable connection time: are you asking about the recommended minimum connection time so that you do not miss your second flight, or how long yours will be depending on what flights are available to Santorini?
Regarding the minimum connection time, I could not find any specifically advertised recommended transit time. However, the different steps of your travel are not hard to figure. As you stay in the Schengen area for your whole trip, you will not cross any border. The airport seems to currently use only one terminal, so that you do not need to switch buildings or walk a long way. The map of the airport shows that you will arrive at one of the 10-15 arrival gates and depart from one of the 10-15 departure gates, the departure ones located just above the arrival ones. There is no scale on the map but from my experience of what a terminal looks like, a wing with 10 gates can be crossed within 5-10 minutes. There is also no apparent security check for connections.
So to determine your minimum connection time, expect 10 to 15 minutes for the plane taxiing, an extra 10 to 15 minutes if you take a bus from the plane to the terminal (departure gates seems to host planes, but not arrival gates - based on my guessing from the map). Then figuring your boarding gate and crossing the airport should take no more than 10-15 minutes.
So overall, less than an hour should be fine. However, any flight can be late, so you should take a margin for that. Based on the board of arrival times, the airport seems to expect more arrival flights in the middle of the day, so if you arrive at that time, the taxiing may be longer.
Overall, planning 1h30 to 2 hours is a reasonable amount that will let you catch your next flight on time. Remember that as you will have two different bookings, so if you miss the second flight because the first was late, you will not be entitled to a refund or a later flight (not for this reason at least). So plan enough time between your flights if you are worried about missing your flight.
If you want to know when flights to Santorini are departing from Athens, Athens airport has a page with its domestic direct flights. It lists the following daily departures to Santorini, operated by Aegean Airlines: 10.00, 15.30 ad 19.10. With these three daily flights, you can find one so that your connection is long enough to leave enough time to connect and not too long (so that you are not bored waiting at the airport).