Turkish visa

Forecasting the time is nearly impossible. I would allow a couple of hours assuming your plane is on time. The visa is a 30 sec transaction at immigrations - Pay $20 US and get the stamp. And maybe 25 not sure. No more than that.

See my post under Turkey just below. It took us about 45 minutes to get from the plane through the visa process. You would probably need at least twice that much time.

I agree that forecasting time is nearly impossible. I say that because in Istanbul we stood in a long line for our visa (US$20) last September, and then a longer line for passport control. The whole thing took almost one and a half hours.
Good luck.

I typically allow about two hours between arrival and my next flight out. Maybe add another 20 minutes if you're sitting in the back of the plane. In addition to the recent experience waiting in the visa lines mentioned by the other posts, one additional factor to consider is the possibility of a flight delay. If you're the type of traveler who doesn't like to be stressed, I'd add another hour to allow for a minor mechanical issue or weather delay on your flight from the U.S. to Istanbul. Who is your carrier between Istanbul and Ankara? I've missed flights on Turkish Air and simply been booked on the next available flight at no charge. It may be worth it to check on the rebooking process and cost with the carrier you plan to use in case you get a delay. Be sure to bring cash (U.S. dollars) for your visa. Good luck!

It doesn't sound like you are going anywhere in Europe first, but just so you know, you can pay in euros for your Turkish visa if you don't have USD. (I did so in November because I had been living in Europe and didn't have any USD to bring with me.) I believe it is 15 euro compared to $20.