How far in advance do you have to plan your vacations so as not to have conflicts with work?

a month or so

Depends on if they will actually give you the time off. Choose a slower time of the year (i.e., not around the holidays) and good luck. I know people who have requested certain time off months in advance and told they would get that off -- only to be on the schedule and working.

24 hours. But sometimes you still receive "Belittling"

I plan mine as far ahead as possible, but at least a couple of months.

4 weeks ahead of time. Just enough so people know they can have more hours for that week which is greatly appreciated for everyone.

a couple of weeks for longer trips

The company tends to be very reasonable about vacation time and there is a fair amount of flexibility.

Months. I wish we could opt to take more unpaid days off of trade a bonus for days off.

I try to to inform as many people and as soon as I'm starting to make the plan.

I’ve never had to worry about this. I just book my vacations when I have a vacation. So long as I don’t book something during a major company event, like Dreamforce - the dates of which are often announced more than a year in advance, it’s never been an issue.

2 weeks or more.

at least one or two weeks notice.

It all depends on your position. 90% of the employees can decide a week or two out. If it's just a day, generally you can let them know the day before, or the day of.