[seiji]

"The first 90 percent of the code accounts for the first 90 percent of the development time. The remaining 10 percent of the code accounts for the other 90 percent of the development time." http://en.wikipedia.org/wiki/Ninety-ninety_rule

You could also go Pareto and say the last 20% takes 80% of your time, so take the time you've spent already and multiply it by 4 to get your remaining time until completion.