Split tasks so they don't take more than a morning or a day.

If you want to know how much a big project will take, you count the tasks, multiply by the half days and there you go.

You can also get a good sense of your throughput when each story is roughly the same.

If a task takes longer or shorter, doesn't matter, it'll bring the throughput of the week down, and next time you're doing this sort of thing you'll likely split it. Just keep counting tasks.

With more sophisticated estimation you'll still get it wrong, so the point is to make the process of planning faster and simpler, since you won't get it right with a bunch of investment anyway.