I've always thought, if you can, with reasonable accuracy, predict how long it will take to build a piece of software, then it's probably already done, and you should just apt-get it or whatever.

However, if it's something truly novel, then kind of by definition, it's impossible to predict how long it will take, because it's never been done before.

Of course, there's a million shades of gray in between, but it's not like bridge building. Bridges are pretty unchanged in 1000's of years. Most software you write, should be brand new, otherwise, why do it?