It's hard to say. In the USA, sewer lines typically slope around 1/4" per foot. Note that larger pipes don't need quite that much slope, and more is okay as well. Normally, where there is a 90-degree angle, there is a cleanout since an auger or snake with a cleaning head on it has trouble sometimes going around that bend. Good design would probably dictate that from the cleanout/inspection it runs fairly straight to the main sewer line.
Tree roots are one common thing that will block up sewer lines. Another is a collapsed or broken line that can happen if any heavy construction equipment was running over the area.