When it takes longer to say the abbreviation rather than the word being abbreviated.
Prime example from last week's Justice League International #8:
Seriously - say Booster's dialogue out loud and count the syllables as you do.
"B.W." = four syllables; "Batwing" = two.
It may take less space on the page to write, but it takes longer to say it. And to compound matters, Booster does it twice in the same issue.
It's a little thing, but it's annoying.