One day is enough if you mainly go to the Great Wall. And I heard there is some one day tour in Beijing including about 3 scenic spots at least. Anyway, from Beijing railway station, you can take the subway line 2 to Jishuitan Station first and then take the bus 919 to Badaling great wall. If your train reaches south station, you can take the bus 20 or taxi to the Beijing Railway station first. It will takes around 2hrs to the great wall.