I would say the reasonable time is whenever she gets back to you? Kidding.
On a serious note though I try to not leave anything in my inbox for longer than 48 hours but I have weeks where I’m booked completely absolutely solid and I know I’m not the only one. Sometimes I have a day a week where I’m able to answer emails.
Here is my advice: If you’re on a tight schedule, try to reach out literally as early as possible. If it’s coming down to the wire and you really can’t wait, you aren’t beholden to the first person you reached out to but it does look flakey. I hope that is helpful.