Collins is famously private, and she almost never discusses her actual workday in interviews. In one official Scholastic Q&A, though, someone asked her directly how she spends a writing day, and she answered in real detail. It's one of the only times on record.

The Direct Answer
She eats cereal, starts writing almost immediately, and counts three to five hours of actual writing, or even a day spent just thinking through the plot, as productive.
The full description comes from a Scholastic 'A Conversation' Q&A distributed with The Hunger Games trilogy press materials.

Cereal, Then the Page

Asked directly about her workday, Collins gave one of the few concrete answers she's ever provided about her physical routine. She keeps the morning simple on purpose. Breakfast is cereal, and she sits down to work as close to immediately as she can manage, because she's said distraction before writing makes it harder to focus once she's actually at the desk.

I grab some cereal and sit down to work as soon as possible. The more distractions I have to deal with before I actually begin writing, the harder focusing on the story becomes. Then I work until I'm tapped out, usually sometime in the early afternoon. If I actually write three to five hours, that's a productive day. Some days all I do is stare at the wall. That can be productive, too, if you're working out character and plot problems. The rest of the time, I walk around with the story slipping in and out of my thoughts.

Suzanne Collins, Scholastic 'A Conversation' Q&A

Three to five hours is the number she names, and she works until she's tapped out rather than to a fixed clock, usually finishing sometime in the early afternoon. Her definition of a productive day is the interesting part. Writing three to five hours counts. So does a day spent staring at the wall, if that time went toward solving a character or plot problem. She measures the day by whether the story moved forward, whether that took the form of finished pages or an afternoon spent untangling a knot in the plot.

The Planning Happens Before the Sentences Do

Collins has also described working out her key structural points before she starts a story: the inciting incident, the acts, the breaks, a mid-story reversal, the crisis, the climax. She's said she leaves the space between those points uncharted on purpose, so her characters have room to develop and she can follow a door if one opens unexpectedly. She's also spoken about relying on a three-act dramatic structure she built during her years as a playwright, decades before The Hunger Games, calling it close to automatic after so much use across ten books.

That combination, a loose morning ritual paired with a fixed structural skeleton underneath it, is likely part of why her books read as tightly paced as they do. See the full Suzanne Collins writing routine for more on where that instinct for structure came from.

What You Can Steal