It’s pretty easy to tell.
Little things like meals (and showers) cease to have the same importance when you’re in the heat of writing an ad.
David Ogilvy recounts spending ten days (10 DAYS!) at home doing nothing else but writing this ad for Puerto Rican Economic Development in 1955.
You can find this ad in the magnificent (now out of print) book, The Art of Writing Advertising, which contains the ad and story.
Ogilvy put his heart into writing this ad — a subject he was “intellectually and emotionally committed to.”
On the flip side, he also describes how excruciating it can be to write an ad for a market you’re not into. For him it was beauty because he was “not terribly interested in lipstick and face powder.”
This ad was part of a series that literally transformed the island in short order — from shanties and sandy beaches to factories and smoke stacks.
Here’s the 148 KB pdf, “Now Puerto Rico Offers 100% Tax Exemption to New Industry.”