I also have a vague memory that I saw "half an hour" before.
Does it mean 90 minutes?:?:
A half hour = AmE
Half an hour = BrE
We Yanks say "half an hour" just as often as "a half hour:"
"I'll meet you in the lobby in half an hour."
"My appointment with the doctor was at 2:00, but after the nurse weighed me and took my blood pressure I waited for half an hour until the doctor finally saw me."