I think the idea of a last-minute trip is incredibly exciting! Whenever I hear of someone doing it, I imagine some young, sophisticated, fancy-free young woman (or man) with cooler friends, cooler luggage, and cooler sunglasses, than I will ever have, and of course, a trendy straw hat...
Unfortunately, with a busy family of four to plan around (including 2 kids who play school and travel sports, sometimes 2-3 at a time, for 9 months out of the year, a husband who works full-time and leads a very active Boy Scout troop with monthly campouts, and me, who also works full-time, gets only 2-3 weeks of vacation a year, and spends all summer on community theatre productions), we are long past the point where we're able to be spontaneous without tearing somebody away from a pre-existing obligation.
That said, our "fastest" WDW vacation (from booking to travel) was a whopping 7 months.
Our fastest non-WDW vacation was about 4 weeks (a road trip to the Adirondacks for a week at an incredible rented lake house), but that was in the middle of a COVID lockdown, when our calendars were wide open!
I think it's awesome that you can "carpe diem," OP, and wish you and your friend an awesome trip! We who cannot do such things envy you.