Corrina, as you come out of international arrivals, you need to walk about 100 yds to your right & the otogar bus stop is there in front of the domestic terminal.
I was on the same bus to Fethiye as Rosy on Friday, the weather was foul over the mountains, so took around 4 hours. The previous Sunday, I caught the 6pm to Antalya, there was no stop & we arrived 2 1/2 hrs after leaving Fethiye, so the journey time can vary quite a lot