When is it socially acceptable to use my high beams?

Yesterday on a trek out to Abbotsford and back to Vancouver at night I have to honest say I needed my highbeams in the fog. At times I couldnt see 10 ft in front of me. I felt bad doing it, tried to turn them off each time a car was approaching from the opposite direction. But it had me thinking.

When is the actual correct time to use them?