at (one's) convenience
When one has the time to do something. No rush, you can fill out that paperwork at your convenience.
at one's convenience
Also, at one's earliest convenience. Whenever one wishes; also, as soon as one can. For example, Pick up the car any time, at your convenience, or We need that drawing very soon, so please finish it at your earliest convenience. The use of convenience in the sense of "ease" or "absence of trouble" dates from about 1700.