What is the expected level of commitment for postdoc positions?
Every position is different. Every PI is different. Every field is different. In math you can probably set your schedule to suit yourself. In some lab sciences with demanding PIs it isn't so easy. But a few points to consider:
First it is fair to explore your working conditions before you start, though it may be a bit risky. If you can manage a visit, you may be able to get a sense of it from other post docs at the institution. But you can also just ask for expectations. You aren't required to damage your health, but some PIs don't know where to draw the line.
Next, you may find that the burnout you experienced and your need for a reduced schedule doesn't continue into the future and it might just go away. Lots of people have such a period, some in the middle of their studies and some shortly after. But a change of scenery, literally, might have a positive effect.
Don't worry about the future too much. Your situation, health, and attitudes may change. But a positive post doc experience could go a long way to helping. Many professors work long hours, but it is because they truly love to do the work. It ceases to be a job and becomes an avocation. What could be better than to be paid to think?
If you take time off, make sure you can explain it as something other than recovery. Have a flexible plan for your return if you decide to do that, and find something to do that keeps you from getting rusty. If you come back, you want to seem enthusiastic about it to anyone who reviews your CV and SoP.