As a developer with some very basic sysadmin knowledge, I'd say you have to have enough sysadmin skill to set up and administer your own system, but not enough to keep it running and deal with attacks and security in the OS. (Obviously, attacks and security in the software you're writing are still your responsibility.)

I say this because if I had to wait for a sysadmin every time I wanted to see if something worked, I'd spend a lot of time doing nothing. And it's likely that I couldn't even solve a lot of problems.

So I think you not only have to know what they do, but some of how to do it.