It really doesn't take long as the national and international terminals are side by side and connected by a walkway. It can be done in an hour if everything goes smoothly, however, there are enough uncertainties (immigration, luggage and delayed flight) that I allow for 3 hours when I am travelling on 2 separate tickets.

I suffered delays of more than an hour on all my flights the last 2 months to and from Istanbul, while my girlfriend waited 3 hours for one of her flights to Izmir. I now only use flexible business tickets from TK for domestic travel in Turkey, and rebook on an earlier flight if possible. Don't expect any help if you have an economy ticket and miss your flight.