In my, once upon a time, programming job, I think we wrote about two or three times as much test code as actual code. Coming from a math background instead of actual programming, that was my biggest surprise.
The percentage of test code required depends on the language. The stronger the typing and the more statically verified it is, the less testing is required. The total "correctness code" is basically the same, but it's moved from dynamically run tests into the compiler, which statically verifies the rules.