I think you really need a whole day to take in Auschwitz. We travelled from Krakow and took the train which was really easy and it takes around 1.5-2 hours to get there, but the bus is also an option.
Auschwitz Birkenau is the site of the gravest mass murder in the history of humanity. It remains a memorial that can serve everyone as a mirror of the human soul and a prism for looking deep within ourselves. One thing that you just cannot get your head around is the fact that so many have died! I would recommend to take the tour as the guides are very impartial and "matter of fact" about the site.
I found that it stirred something deep within me and I am not ashamed to admit I had a little cry.
If you are looking for things todo whilst visiting Poland then this is a must!