If you're looking for a direct opposite in terms of urgency:
* Later
* Whenever
* At your convenience
* No rush
* When you have time
If you're looking for a more nuanced opposite:
* At a specific time (e.g., "by Friday")
* When possible
* When convenient
* As soon as practical
The best opposite will depend on the specific situation and the level of urgency you want to convey.