There is only about 1 block published every 10 minutes on average. It is a lottery to determine who processes them and gets the 50 BTC block reward (plus transaction fees).
I entered 600000 khash/s into the
Old Bitcoin Generation Calculator. At the current difficulty, you can expect:
- On average, you will process one block every 47 days (assuming no difficulty increases)
- 50% probability of processing a block within 32 days.
- 95% probability of processing a block within 140 days.
The
New Bitcion Mining Calculator assumes you are mining as part of a pool. A mining pool splits the 50 BTC block reward among participants.
If it was me, I would run solo anyway. Your 95% probability is less than a year, so I think you can expect to process a block within a year, even with difficulty increases. Of course, I plan on solo mining (in a limited way) even with an expected 50,000 days before processing a transaction.

He's got 2 6990's, so he's mining at double that rate (so, halve those estimates).
Everything on that screen shows your machine is working properly; you just haven't found a coin yet. Your choices are to (a) wait patiently, and be really excited when 50 BTC shows up in the next few weeks -- nothing to sniff at in today's market! -- or (b) modify the mining scripts to point at one of the many mining pools, so that you can at least see money accumulating on a daily basis.