The only consistently accurate estimates are based on previous work for which you have data. As a general rule, if you did X and it took Y days, the next time y

The only consistently accurate estimates are based on previous work for which you have data. As a general rule, if you did X and it took Y days, the next time you do something similar to X, it will take an amount of time very similar to Y. The problem is that (a) most organizations don't track and record the time to do anything and (b) people don't want to take the time to break down tasks to units similar to something they did before. Estimation is itself a time-consuming task.