Sometimes, it's as easy as mowing the yard, going shopping, or just heading to the library. No one to tell you what to do, and it's YOUR time. Others just simply go camping with the guys, or have a girls' night out. Feeling burned out isn't unusual and it's not the death of your marriage. Take some time off from each other, and it renews your focus on the marriage you have.
Distance makes the heart grow fonder, so they say.
For some, it could be a monthy necessary, for others, once a year, or twice a year. Your relationship dictates when you've got to let off some steam. You are not running away from your marriage, but strengthening it. Just as you got away from your parents from time to time, so do you need a breather from your spouse. It's become so unspontaneous, even your mother knows when to call you.