There are a variety of synonyms that can be used for the phrase "when one has time." Some options include "at one's leisure," "in due time," "ultimately," "eventually," "in between other tasks," or "when the opportunity arises." Additionally, other phrases such as "when there is a lull in activity," "when things calm down," or "when the coast is clear," can convey a similar meaning. Depending on the context, some synonyms may be more appropriate than others, but any of these alternatives can help to communicate that something will be done when there is time available.