Make a guess, double the number, and then move to the next larger unit of time.
POUL-HENNING KAMP
When I started in computing more than a quarter of a century ago, a kind elder colleague gave me a rule of thumb for estimating when I would have finished a task properly: make a guess, double the number, and then move to the next larger unit of time.
This rule scales tasks in a very interesting way: a one-minute task explodes by a factor of 120 to take two hours. A one-hour job explodes by “only” a factor 48 to take two days, while a one-day job grows by a factor of 14 to take two weeks.
The sweet spot is a one-week task, which becomes only eight times longer, but then it gets worse again: a one-month job takes 24 times longer when it is finished two years from now…