Sorry but couldn't resist..
assuming you had an Nvidia GTX680 (120MH/s)..
it would take
580 years to solo hash a block on average.
Seriously don't waste your time and electricity on this!
If you really want to mine on a GPU first thing you should do is sell the Nvidia, and use the money to buy the equivilent AMD/Ati 7970 (640MH/s) it's 5 times faster!
Then seriously consider Litecoin instead.