How much (percent) does programming involve digging into other people's code and reading it?

I have someone fairly new on my team for a software that's been up and released for over two years. We add features to it and fix bugs. They are extremely frustrated with having to read code (instead code comments) and debugging.

So I pose this questions, What percent of the job of a "programmer" in general involves digging into other people's code and debugging\reading it?