At somebody’s convenience
Definition 1 of 1
Pro Tip 1/3Flexibility Offered
Using 'at your convenience' communicates that the person can choose the time that works best for them. 
I've left the documents on your desk for you to review at your convenience.
Pro Tip 2/3Politeness
This phrase is polite and shows respect for the other person's schedule and priorities. 
Would you be able to provide feedback at your convenience?
Pro Tip 3/3Non-Urgency
It suggests the matter is not urgent and can be dealt with when the person has spare time. 
There's no rush, so please get back to me at your convenience.