What's considered a last minute trip to you?

A trip that was not on your radar until all of a sudden.

For me: anything less than 6 months (I do like to plan way, way ahead and then tweak the itinerary along the way).

To me, that is one that happens within hours of a decision to go.
I used to run home, grab a change of clothes and head to the airport. I'd leave a note for my wife telling her where I was headed.
There was one time that I went to Korea for what was supposed to be a short stay. I left with one day of planning.
While I was there, I realized that I also needed to be in Singapore. I went there, stayed a week and then went back to Korea. After another week in Korea, I had run out of my prescriptions......I had not planned on being gone that long. It was interesting getting that sorted out. It turned out that the company I was visiting had an onsite pharmacy. Once I got my slow poke physician to issue scripts, they were able to take care of my needs. Before returning home, I made one more quick trip to Singapore.
Often, I was using my computer bag as my luggage. there were many times when I bought clothing in transit. I think I still have a shirt I bought in the Hong Kong ferry terminal.
If you have a few weeks to plan, that is not what I would consider to be "last minute".

Last minute means it is in the next month or so.

The last "last minute" trip was decided three hours before my flight.

When I'm traveling, quite often, I'll make changes along the way.

I did one today for 12 days from now. Canceled a long multi-change train trip for a short train trip and a short flight instead.

12:58 vs 12:59. ?

What time is it now?

But realistically,

Within my country?

Air or road trip?

Meet me this weekend? By air, road, ???

I used to answer interview questions about availability to travel with, "I'll need enough time to board my pets and pack a bag."

If I'm paying the fare, how much?

But you are asking about Europe for leisure. So, can I find affordable transportation? Tie up loose ends at home/work?

Have I already planned that money for another trip?

Do I really want to go?

Since I usually plan a year in advance, last minute would be 6 months or less.

I could leave for Florence, Rome, Venice, or London tomorrow if someone could puppy-sit our “large personality” dog. Our cabin would take a couple days to organize, mostly to get the food we would want. Otherwise, I would need a week for Europe. We have well-honed lists. And Google. But normally, I’d be planning 6 months in advance in order to get the flights and apartments we want.

I tend to firm up loose plans in the week before we travel and often cancel due to weather or medical, so a last minute trip would be a "let's go tomorrow" one.

Six months planning? I’ve gotten up in the morning and unexpectedly been across the country by dinner in LAX for a civilian business event. Before retirement came I woke up on a Monday in Europe and was at happy hour in Muscat, Oman on Wednesday.

Everyone has different levels of tolerance so its impossible to judge what is last minute.

When we decide we want to get away, buy tickets the next day, and leave within a few days.

They have last minute trips from the Frankfurt airport. You show up and pick a few choices (Italy, France, etc) and off you go. One of my friends did this last month and had a great trip to a city she had never thought about visiting. She wants to do it again. I have seen them advertised and thought it would be a fun weekend thing to do.

Oh, wow, Ms. Jo, that would actually be really fun.