A few things to clear up, as I was confused by reviews:
You MUST take a guided tour (40 polish dollars or so) if you arrive by yourself before 10 am
It takes around 4-5 hours from start to finish, and there is a lot of walking so be prepared
I took the train from the main station, which cost about 28 polish dollars return, and about 1.5-2 hours. It was a nice train ride through the Polish countryside. Upon arrival, I walked to the main museum (about 2 kilometers, 15 min walk).
My tour guide was great. Knowledgeable and easy to ask questions. If I had gone solo, I wouldn't have stayed as long, but I never felt rushed as some other reviews have suggested.
I thought Dachau was set up better, but I'm glad I saw Auschwitz and the sheer horror and scope of it.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC