How long to you do deep work, and how frequently? Do you take breaks? What do you do during those breaks?

I've personally found the pomodoro method works very well for me. 25 minutes on, 5 minutes off. Every fourth break is 15 minutes.

I only do one 45 m session per day, because I usually don't need that much work done that requires intense focus. In an 8 hour day I work like a marathon, not a sprint. During the breaks I talk to people around me, or if I work from home, I relax in bed. The bed proximity does wonders for long term fatigue.

Since I am self employed, I do deep work/ time blocking. Quite frequently.

I do time blocking the first thing when I wake up,after I meditate and before I get out of bed.
It looks somewhat like this.

Wake up 3:30
Meditate 3:45 or longer
After that i block out time for 30 minutes to write + edit for the coming day.

Next wash up
Then continue the day.

Other tips that help is getting ready by set cothes out the the night before .

I am getting better at this as I go with being self employed.

It is best to take your deep work seriously and breaks too.
I break ever 15 minutes with writing and drawing and games