We live in Houston, so it's something we have to plan in advance due to PTO, etc.
Our first trip, we stayed 9 days so I planned a year (14 months) in advance and I'm so glad I had the time to do so. Now, we plan about 2-4 months in advance, never less than 180 days (for ADR reasons), based on how long we're going and our budget. But now I know what to do, what interests us, etc. This is just for WDW; for any other parks I would do the same (plan longer in advance for my first time or depending on budget).