Lots of people seem to be saying that this won't unlock in endless mode. When going for this achievement, do it in story mode just to be safe!!
If you have your TA settings set to show "Unlocked achievement description," and you can't see the requirement as a result, this achievement is for spending 1 hour in latte art mode.
Just as an FYI, though, this didn't unlock for me until after I served the drink, despite having let it sit for over 2 hours.
Hopefully I can save somebody some time! :)
Also, latte art is not available for all drinks - it must be a latte (obviously lol). The easiest one is coffee, milk, milk. After it's made, select latte art on the same screen where you would otherwise serve it. Pour some milk in and just let it sit - seems like time doesn't start counting until you pour milk, so just sitting in the screen with a plain, black coffee won't work.
You don't actually have to do anything to the "art" for the time to count. That said, it's fun to mess around with and see what kind of abominations you can come up with!