I have a quad core i7 920 cpu overclocked to 3.6 GHz and have been running the bitcoin client for about a years time now.  I definitely don't run my computer 24/7, but I would say I average at least 3 hours per day every day 365 days a year.  The priority of the client is set to high, and the "Generate Bitcoins" thing is enabled as well.

Is this normal?  I'm sure people ask this all the time, but I feel like I must be doing something wrong to have not even generated a single bitcoin so far.