I am quite new and wondering how long it would take with a single GTX 680? EDIT: I am using a pool.
For solo mining with a single GTX 680, which will produce 120 Mhashes/s, you would expect to find one block approximately every 98 years.
At the current difficulty, solo mining is inadvisable under practically all circumstances. Pooled mining should be done instead.
Furthermore, mining utilizing graphics cards is rapidly losing viability with the release of FPGA and ASIC-based mining hardware, which operates at far greater efficiency. Unless you have exceptionally cheap electricity or have a use for the heat output from the graphics card(s) (e.g. heating your house for winter), graphics card mining is unlikely to be profitable for you.
Additionally, mining with NVIDIA graphics cards has pretty much never been worthwhile, as they are far slower and less efficient than comparable ATI/AMD hardware, conflating the above issues with graphics card mining.
You will never mine your own block while in a pool. The owner of the pool owns all the blocks mined. Your best bet is to buy some ASIC (butterfly labs?) box and let it roll. You could get lucky.
Two years ago I solo'ed with a Raedon card and got luck after about 4 months.
I think with such a tiny amount of hashing power even being in a pool is not worth it.
But again, you will never mine a block while in a pool, only part of a block.
So if you make 0.000000485032112 bitcoin per minute, and let's say 1 bitcoin is $1000. So your making $0.000485032112 per min, or $0.02910192672 per hour or ~$0.69 per day, but the power costs of running the PC don't make it worthwhile.
1) The time where GPU mining was financially profitable has been over for several months already. (Unless you don't pay for your electricity.)
2) Nvidia Cards are even less efficient than AMD cards for mining.
3) Solo-Mining is even more unlikely to succeed.
You might want to consider mining Litecoin or Altcoins, but even there the competition is slowly ramping up.
It took ages for me to mine bitcoins because of enormous competent people mining with excellent machines. You better get a Mining hardware to make your time worth since you will eventually end up paying your internet and electricity bills.
Even if you are in solo or pool mining, for the current difficulty (2.4122720023e+11), you will need at least 12Ph/s to gather a full bitcoin block (12.5 bitcoins) in 24 hours.