[All of the things she could say in response to that sound a bit too trite, but saying nothing would probably be worse. So, after about a second sorting through the answers, she picks the one that sounds least trite:]
Anytime.
[.......And then she leaves without ever telling him that she's going on a mission herself and thus probably won't ever get a good moment to mention it and it'll come up someday and he'll be surprised. But she's not going to go out of her way to tell him when he's got enough to worry about as it is, especially when she doesn't realize how long he'll be gone.]
no subject
Anytime.
[.......And then she leaves without ever telling him that she's going on a mission herself and thus probably won't ever get a good moment to mention it and it'll come up someday and he'll be surprised. But she's not going to go out of her way to tell him when he's got enough to worry about as it is, especially when she doesn't realize how long he'll be gone.]