If you have the flexibility to restrict your work to 20 hr/wk you probably also get to decide when you work; nothing says you can't work 20 hrs straight, or 80

If you have the flexibility to restrict your work to 20 hr/wk you probably also get to decide when you work; nothing says you can't work 20 hrs straight, or 80 hrs/week for a month and then take a break. Lots of consulting-type work actually prefers this approach.