Anything over 4 hours and we're both in F/J one way or another. Anything under that and we still might be if it's cheap enough. When traveling with friends (either together or just her) we will sometimes burn miles to book the whole group in FC.
My father-in-law worked for DL corporate back in the day, so Mrs. Lee grew up with premium cabin flight benefits. Unfortunately he did not get to keep them after he was laid off.