I think some races on Amateur Level have 5 laps. I also think that any race that has at least 5 laps has the tire wear on. Everything in Professional has tire wear activated, but at least 6 laps on mediums tires is a good standard to follow. Different cars wear tires faster than others. Your style of driving also affects how fast your tires wear down.
When I normally race, I am pretty aggressive. I make most corners as fast I can possibly make them. When racing against the F1's, I am pitting just as much as they are usually. Also I pit about the same time they do, so I have gotten pretty good at predicting when to pit, since that will help me have a partner to draft with.....