>>behnam+(OP)
This is good advice but forgets that people are part of how long time something will take as well. If it's one engineer, it'll probably take $estimation * π, but if it's two engineers, you probably need to double that. If it's three, triple it. New calculation should be something like "($estimation * π) * $peopleWorkingOnSameThing". Communication overhead and mistakes should not be underestimated.