Are there other ways to say Take your time?
- Whenever you can
- It's not (terribly) urgent
- No rush
Any other ideas?
"at a time suitable to you"1
Another turn of phrase is (see this question and its comments for information about the use of this phrase):
At a time of your choosing
1 "Definition of 'at Your Convenience'." At Your Convenience Definition and Meaning | Collins English Dictionary. Accessed March 25, 2018. https://www.collinsdictionary.com/dictionary/english/at-your-convenience.
Here is a letter: read it at your leisure.
From A New and Complete Concordance Or Verbal Index to Words, Phrases, & Passages ... ~ By John Bartlett
At your pleasure
at someone's pleasure
As and when someone wishes.
‘the landlord could terminate the agreement at his pleasure’
Collins Living Dictionaries
There is no hurry (or just no hurry) is a good colloquial expression that can also be used in a business setting:
Pay me back whenever you can. There is no hurry.
Since this question is tagged with "business-language," I'm going to assume you're looking for practical solutions rather than making a collection of colorful and unique phrases which mean "no rush."
First, I would avoid uses of phrases such as the ones you listed. A minor reason is that they're overused and clichéd. (That's possibly why you posed this question.) But a more important reason is that they are not helpful. They say what the request is not. They don't say what the request is. They can be ambiguous. You may feel they are polite, but they are not because they don't help the recipient actually plan for the request.
My personal preference is to always be as specific as possible. If you have an externally-imposed deadline, state it. If you have a goal date in mind, ask if that's possible. At the very least, as the recipient when they think they could deliver. Even in situations where there is no deadline, the work probably has some kind of shelf-life. For example, if they literally got the work to you 25 years from now, could you still use it? If the answer is no, then it's a best practice to state the actual date you have in mind. For example:
Externally imposed deadline:
The foundation has set a deadline of May 1 of next year. I should need no more than 2 months to finish the preliminaries. Would you be able to send me your response by March 1?
I'd like to get this finished by, say, 6 months from now, and I'd need about 2 weeks to work in your information. Does that give you enough time?
Something truly open-ended:
I have no date in mind to get this done, but if you have an idea how much time you would need, could you let me know? That will help me plan.
Finally, think twice about making a request that is truly far in the future. "Far" will depend on the pace and workload of the organizations you're both working in. But making a request too far ahead is just begging for the recipient to put the task at the bottom of their To Do pile and to forget about it. If you really want to give as much notice as possible, state that fact, and mention that you'll send more information later.
May 1 of next year is a long way off, so this mail is just to see if you might be available. If so, let me know and I'll send more information closer to the actual due-date.
bide (one's) time TFD
To delay some action until an ideal moment or situation reveals itself.
"You may bide you time on ..." Bide your time until a crash to enter market? business article