Unit tests: How much is enough?

Looking back on some of my earlier projects, I noticed that  UTs account for roughly 20% of my total code lines.   I was wondering if anyone else has similar numbers for their unit tests?   It seems to me that my numbers are a bit low,  to achieve total test coverage I would expect over 50% (meaning more UT  code then project code). On the other hand, I have yet to come across an application which actually achieved that kind of testing completeness other than certain open source libraries.  

I know what the literature says,  now can anyone come up with some real world numbers to back up my theory?