I think once a week. I choose this for myself as a Christian because of the model provided in the bible. It can motivate you to stop procrastinating, for instance, if you have a deadline for Monday but rest on Sunday. So, you'll get it done by Saturday, rest Sunday, and be recharged for the week or work week. Any day will do. Make it work for you, don't work for it.
It's recommended to take a 3 to 5 day full rest day but I suggest you do however long you feel you need to rest. Rather that be a day, 2 days, or a week. I hope this helped.