Hello Shawtay!
The location where you can find the Sun-Marked Goanna is on the South-West of Killmaresh at the Southern Steppe, right here on the Red Circle:
I remember when I helped my friends sometime ago to finish this mission, we had to wait around 5 minutes to it appears once again. As a tip you can lured it to the little bridge so you don't have to face a lot of goannas that appears on that place. Also if you don't want to face the goannas you can also use fire and energy bombs and make your own way to the east where the Sun-Marked Goanna is located.
I have no clue about is the respawn have any kind of block time. I was hunting there with friend though and we kill this so many times but I didn't pay attention on the Tibia Time. I can check it later for you.