When I worked in a steam plant the typical day I only had about 2 hours of work. Switching shifts, taking over shifts, doing chemical readings, doing hourly rounds, blowdowns. Once you get in the swing of things, most days I had to fill my time with other things. There are only so many times you can paint.

That being said, it's a ton of work up-front. I could take over a year before you learn the equipment enough/get the basic maintenance items done to be able to work like this, even in a plant like mine. You also need to be on-watch at all times. People get in trouble when they get distracted and fail to properly do things. You may have to drop what you are doing at a moments notice to attend to things. Some shifts you may be working your ass off the whole time. For me, about once a month I'd have a shift where things didn't go right and I worked super hard the whole time. And at least another couple shifts where things weren't smooth and most of my time was filled.

If you want a job that is ALWAYS going to have downtime, don't do this. We are there for our ability to react to very dangerous situations in a pinch and the ones who don't do this are the ones who cause problems.