I remember I once read in Wikipedia about a law stating that the actual delivering time will always be 4 times less to 4 times more from the estimated time, even when you aware that law and know every detail in the project. What is that concept so I can look it up again?
It sounds like the Cone of Uncertainty. The idea is that at the beginning of a project or effort, there is enough uncertainty that estimates are generally in the range of being 4x over or 1/4 of the time it will take to complete. As the project progresses, these uncertainties begin to be resolved or better understood and the range of estimation becomes better.
Steve McConnell writes about this concept in Rapid Development and Software Project Survival Guide. It's also discussed on the Construx (McConnell's company) website. The idea in software development, however, originated with Barry Boehm in Software Engineering Economics.
Cone of Uncertainty
Research in the software industry on the Cone of Uncertainty stated that in the beginning of the project life cycle (i.e. before gathering of requirements) estimates have in general an uncertainty of factor 4 on both the high side and the low side (Boehm 1981). This means that the actual effort or scope can be 4 times or 1/4 of the first estimates.