My (ultimate) answer would be "
I expect you to tell me that, at least at first, until I have experience enough to decide for myself".
Or, are they planning to throw you in at the deep end, without any help?
I imagine that it is as @Bernhard said in a comment above
there is no one good answer. This is a question to show that you understand the role. The reasoning matters, not the number.
So, I would show them my thought process, as I do for just about all interview questions, then throw it back at them - "
... that's what I think and why I think it, but what's the reality here?".
After all, you are interviewing them too, and deserve a number to help you to decide whether to accept an offer.
[Edit] why are they so concerned about coding? It's the shortest part of the development process. Perhaps they mean time spent with your team, and don't consider upfront requirements analysis, & architecture definition to fall into that category?
But I am concerned that they don't ask how much you will participate in reviewing - don't they do any?
And while the ratio of testing to coding may vary, I would expect it always to be more time for testing. I see it as a red flag that they don't ask about testing.
It sound like they had a rather one dimensional view of the development process, and see it all as "the coding". Worth thinking about.