I've set Boinc to use up to 100% and I'm running the climateprediction.net project. Each of the four instances that the BOINC manager has created show time remaining in the thousands of hours. 75 days per work unit can't be right I thought so I checked my CPU usage and boinc is using 0.20% and BOINCManager is using 2%.

This is on a Mac Pro 2.66 running OS X 10.4.9.

I'm not doing anything particularly demanding at the moment, just browsing the internet. I have around 90% unused CPU usage.

75 days per workunit is absolutely right. I'm running 4 CPDN workunits on a Dell Windows workstation (along with SETI and Einstein). My quads only run at 1.86Ghz, so my 110 day estimates (at 50+% done) are in line with yours.

Again, BOINC using 0.20% and BOINCManager using 2% is absolutely right. BOINC doesn't actually do anything at all (like all bosses, it directs and controls other workers, but contributes nothing at all to productivity). The real question is, how much CPU %age do the CPDN worker applications get when you're not clockwatching? As others have said, BOINC might be telling the workers to down tools and make the place look tidy while the owner (you!) is around.