Target scores are divided by points that can be used when applying for Australian / New Zealand immigration. The below list also indicates the general required amount of preparation time. That said, PTE is a unique test because of many factors, you do not need to just fully trust the list. Of course, if you live in any English speaking countries for a long period of time or you must speak English on a daily basis, the required time will vary.
Based on my experience in teaching PTE Academic to over 50 students, this is how I classify the difficulties.
There is not much difference between 50 points and 58 points. 65 points requires practice on more question types, but still not far more difficult than the two. But 79 points is a completely different story. Basically, you will need to cover all the question types for this score and you will need to be careful about everything, otherwise you might fail just by missing a few points which is typical for people aiming for 79 points.