How do you plan your vacations? And how long does it typically take?

It depends where I am going but I've learned over time to travel spontaneously.

As a motortourist, for instance, I would usually make hotel reservations a night ahead. I don't want to show up in a city and not know where I'm staying but I like the flexibility of deciding where I go the next day. I particularly like driving to the outer edge of a regional rail system and then depending on transit at the core. I've gone on many weekend trips on a whim to go to a music festival or ride on a sailboat or see a rugby game, etc.

I do have some big trips that I think about which I guess I'd have to block out a few days to a few weeks and get a flight. I'm interested in (1) hiking in the Blue Mountains of Jamaica, and (2) skiing in Japan both of which involve an element of conditioning (I'd better buy the seasons' pass at Greek Peak if I want do do the latter) Either way it is like building up for an athletic event for a few months. Either one involves a lot of research, in the first case I think I'd have a very definite plan of how I am getting to the mountains. On the other hand I know spontaneous travel is pretty easy in Japan even though I'd probably have at least one mountain in mind as a destination but still have the possibility of a major change in plans.

I probably have a mental list of places I'd like to visit. I also have a set of weeks I'm willing and able to take off.

Then it's a combination of flights.google.com to check for less expensive travel fare for those destinations in those weeks, followed by also googling for nicer hotels in those areas.

Depending on the trip, I might consider airbnb, like if it's a skiing trip or just some little beach town destination, but it doesn't apply to all destinations.

Then once I've landed on 2-3 ideas, I also check if I can get those flights for miles rather than dollars. There's some correlation between "cheaper fares" and "lower miles" but I can't use google.flights.com to search directly with miles, so I use fares as a proxy. If no good deals on miles, then I just pay with hard earned dollars...

It's an involved process, so I will spend some time on it, advance a little, discuss with my wife, sleep on it, come back to it later... it might take me up to a week to hammer out the best candidates, sometimes longer.

My vacations are almost always wilderness camping/hiking trips. Planning and readying for them takes me about a week, but I can get it done in a couple of days if I need to.

If I'm doing a more traditional vacation, it's still about a week of planning all told, but I prefer to get all that planning (and booking hotels, airplane tickets, etc.) done at least a couple of months in advance.