Controlling variables in homemade yogurt
The initial heating stage takes a little time to denature the water soluble proteins in the milk.

The hotter your milk, the faster the process. At 180 it takes 30 minutes. At 190 it takes 15 or so. Heating the milk longer than this won't improve the texture. I don't believe the texture will change very much unless too much water is lost.

Heating for a shorter time than this will make the yogurt significantly more fragile and less creamy.

"The milk mixture is pasteurized at 185°F (85°C) for 30 minutes or at 203°F (95°C) for 10 minutes."
http://www.milkfacts.info/Milk%20Processing/Yogurt%20Production.htm