Give up on time units. Estimate of a task's duration should look like this:
- School exercise
- School homework
- School project
- Small contract
- Big contract
- Manhattan project
And an estimate of a task's difficulty should look like this:
- Solved problem
- School exercise
- We need a proof of concept
- Open question
If someone insists on time units, try to stick to this:
- About a <unit>
- A couple <units>
- Several <units>
Where <unit> is minute, hour, day, week, month, year, or decade.
At this point, those who insist on anything more precise are asking for it.
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