This is what I was taught: double the estimate, and increase to the next unit. It was mostly as a joke, though.
The wise scheme, as has been pointed out, is to adjust predictions during a project. If the task that was initially planned to take a day is routinely taking 2 days (or 2 hours), adjust future plans accordingly.
Higher-level managers are sometimes unhappy with this scheme, while middle-level ones value the accuracy of such predictor-corrector schemes.
This gets us into a related topic of the depth of management structures, and I think the military scheme (of each person directing a roughly fixed number of persons, so the number of levels is proportional to the log of workforce size) might be worth considering.
The wise scheme, as has been pointed out, is to adjust predictions during a project. If the task that was initially planned to take a day is routinely taking 2 days (or 2 hours), adjust future plans accordingly.
Higher-level managers are sometimes unhappy with this scheme, while middle-level ones value the accuracy of such predictor-corrector schemes.
This gets us into a related topic of the depth of management structures, and I think the military scheme (of each person directing a roughly fixed number of persons, so the number of levels is proportional to the log of workforce size) might be worth considering.