Try it here, easy to understand:
http://bitcoinwisdom.com/bitcoin/difficultyIn case of today difficulty at 609,482,679, you should be able to create 0.06876BTC/hour, which mean you will need about 14-15 hour to create 1BTC.
Or for 1 day-24 hour mining you can create 1.65BTC.
Depend on delivery date end of march 2014, and a average expected difficulty raise to 5,941,344,136 the amount will be much lesser.
In march 2014 you can mining 0.007054BTC/hour or 0.1693BTC/day, so you will need more then 6 days already for one (!!) BTC!