Im US, in Hong Kong and want to go to Beijing just for a day, do I need Visa? how long will it take?

Yes, Roe, you need to apply a China L visa at the Office of the Commission of the Ministry of Foreign Affairs of P.R.C. in HKASAR. For regular service, you can obtain visa on the fourth working; for urgent service, you can get the visa on the third working day; for express service, you can get the visa on the second working day.