in (one's) spare time
When one is not engaged in work, school, or other tasks or responsibilities. Ed's decided that he'd like to learn the saxophone in his spare time. Compared to most sixteen-year-olds, Sarah is a walking dictionary. She must read a lot in her spare time.
in one's spare time
in one's extra time; in the time not reserved for work or doing something else. I write novels in my spare time. I'll try to paint the house in my spare time.