Assert(LOC(Test) ~= LOC(App))

Andrew Binstock discusses a talk with Agitar about "how many unit tests are enough?" The upshot is that if the amount of test code is roughly equal to the amount of application code, that generally translates into code coverage of around 70% and is generally "pretty good shape."

I think that's probably about right, although I'll admit to rarely maintaining that level in a serious project -- shame on me.