Short milestones mean small surprises

I like trying to build things through a series of very short milestones. Mostly this is because the longer the milestone, the longer it takes before you can realize that you're going to miss it. It's hard to tell in the first few weeks that a month long milestone isn't going to work out. It's usually in the last couple of weeks that you realize things are taking longer than planned, the technology doesn't work, or the product UI is clumsy.

I personally like one day milestones. It just feels long enough to get something measurable done but short enough to quickly realize if things are going off the tracks. It's also motivating for everyone to see daily progress when working toward big long term goals.