" The cone of uncertainty shows that during the feasibility phase of a project a schedule estimate is typically as far off as 60% to 160%. That is, a project expected to take 20 weeks could take anywhere from 12 to 32 weeks. After the requirements are written, the estimate might still be off +/- 15% in either direction. So an estimate of 20 weeks means work that takes 17 to 23 weeks. Figure 1.1. The cone of uncertainty narrows as the project progresses. "

― Mike Cohn , Agile Estimating and Planning