How long does it usually take to mine a bitcoin block? 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.