Wikipedia has a good, and accurate, discussion on the subject.http://en.wikipedia.org/wiki/Line_level
What you'll find is there is no single voltage or impedance that constitutes line level. I've been used to thinking in terms of 1v being line level, but the actual value is really not relevant. Some devices (like the computer sound cards) allow you to vary the voltage level out. Some audiphile equipment, however, doesn't.
Sort of like asking "how much power does a radio put out?" Same answer...depends...
Pretend it's 1 volt at 10kohms and you'll not get in much trouble.
73 de ACØDQ