The Private Pilot ground school is comparable to a complex upper division college level course. Pilots are expected to know a lot about a lot of different subjects. King Schools PPL Ground School & Test Prep Course takes several weeks to complete, review, study, review again, and study again, but it very successfully prepared me for the knowledge test. I made a 97% on the test, and feel like I will be able to remember most of the material as I progress with my flying.
If you prefer to learn at your own pace, this will definitely work for you.
Private Pilot Online Ground School & Test Prep