Student or Learner
No haste; respond as far as your time allows.
Is it OK?
Beginning with the second part; you could say, "respond as your time allows". This is not a common phrase, but I would understand what you meant. Similarly, when you start with "No haste", I understand your intent, but this is not usually the way we say this.
There's more than one way around this, but I think one of the more common phrases would be:
"No hurry; respond when you can". You could also start with, "No rush". Both are common. You could end this with, "respond at your convenience", which is a little formal.
"No rush; get back to me whenever". This is very common, very informal, and should end with "... you can". Informal speech often breaks the rules, however.