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Don't do too much, or you will waste time and efforts
(ttrpg.network)
You can of course plan the big lines of the campaign, but the more precise you get and far ahead of the present, the more you will either lose or railroad to not lose. Both suck
The only way I'm planning 3 sessions in advance (beyond very broad strokes) is if the players have spent the entire session being unjustifiably paranoid of a random NPC who was never meant to be more than set dressing, rather than exploring what is presently prepared for them, hence what I actually meant to happen gets pushed back. So, all the time.