If she's potty training now (mid-July), I would hope she'd be trained by September. If she's not trained before then, IMO, then potty training was started too early. If you wait til a child is ready, it shouldn't take that long.
My DD trained in one day, & DS took about a week...but I waited until they were ready. DD was almost 3yo, & DS was 3years and 3 months. Honestly, I'd try training her now but is she isn't trained by August...I'd just wait until after the trip.
Just make sure you carry extra clothes with you in Disney!
**Have a great time!