Warning: stream of consciousness type post follows. Exact wording should probably be cleaned up before sending anything to your friend if you find any of this valuable enough to use.
I can think of several approximate wordings you can go with, depending on what the two of you would like to do when you visit (such as, do you want to just hang out with her without doing anything special/sight-seeing-ly, do you want to look into if there are local events or sightseeing attractions (or already know there are and want to visit them), would either be fine with you, or haven't decided yet?).
Whichever of the above applies, I'd say to acknowledge that you understand completely, as it is a busy time of year, and maybe suggest that a couple days after Thanksgiving and before the Christmas/holiday crunch begins that she take a look at the days that you have vacation time, and see which ones would work for her. (This is assuming that you're in the US, if not just do the "before the Christmas/holiday crunch" part.)
Then, if you and she both want to just hang out, you can just request that she let you know before X date because you need to make travel arrangements.
If you and she both want to visit local attractions, ask her to also let you know if which ones in particular that she wants to visit and when, and if she can suggest a good place for you to research her area online to see if there are particular attractions you want to visit. Include both of these, so she is aware you're not expecting her to do all the research alone, but also that you don't expect to call all the shots without letting her give input (and that you aren't going to do all the research alone either). And also that you'd like to know the dates best for her by date X so that you can look for fun events on those dates, and make travel arrangements.
If you don't have a preference or it hasn't been discussed yet, ask if she wants to just hang out or do touristy stuff. And if she wants to do touristly stuff (see last paragraph) and if she doesn't, you'd just need to know before X date because you need to make travel arrangements.