If you're giving hard estimates further than 3 months out, you're probably making it up.
We can and should break large efforts down into phases or milestones. Those should target 3 months or less so the estimates can be meaningful.
Getting good at swaging these 3 months milestones means you can give fuzzy estimates about large efforts. 4-5 milestones means "this could easily take 18 months".
Korny