Pretty much once a book. Glinda is a master of these.
From the second book: Nick, Scarecrow, Tip, and Jack are stuck in a jackdaw's nest. The jackdaws - massive, nasty birds - come as they're trying to escape and attack. Nick holds his ground, breaks out the axe, and fights "like a Roman gladiator" for his friends' safety. Seriously, our Tin Man is a softie, but do not piss him off.