How early should I be at the Marco Polo Airport in Venice for a departure flight?
You need to specify if you're flying domestic or international.
I am flying to Istanbul, Turkey; so Internationally
How about looking at the airport website for each city you're asking about? It took me 2 min to find this.. http://www.veniceairport.it/page/servizipass/traveltips/pre-partenza?m=2740210
We flew last week from Marco Polo to Paris. We arrived two and a half hours early. It was too early. They wouldn't even let us drop our bags until two hours before our flight was scheduled to leave. It was a very easy airport to manage, so I wouldn't sweat getting there any earlier than that. We had a ton of time to kill after we got through security. Another opportunity for Prosecco!
Check in for flights outside the Schengen Zone closes 45 min. prior to departure. I think 1h 30 min arrival at the airport is more than sufficient for a flight to IST. You'll be flying on a B737 (or A320 if it's the 14:20 departure), therefore it's not a huge jumbo jet that has a lot of people at the check in.
Take the advice of your airline, they know the conditions at individual airports. As an additional thought it does depend on how many flights are being processed at the time. These days I find that going through security can take longer than check in. Also check whether you will go through passport control. When we fly from Italy to the UK, which is not party to the Schengen Treaty, we have to go through Passport Control which can also eat up more time.