Don't make estimates. Don't listen to estimates.

"Professional forecasters offer highly variable predictions about likely sales of new product, likely growth in the unemployment rate, the likelihood of bankruptcy for troubled companies, and justified everything else. Not only do they disagree with each other, but they also disagree with themselves. For example when the same software developers were asked on two separate days to estimate the completion time for the same task, they hours they projected differed by 71% on average."

-Noise, by Danny Kahneman et al