- I think it will take a week, but this part looks a bit complex, so we may have delays.
- Or, I did something similar last month, so I am quite sure that will take 3 days.
A good PM will know how to deal with this information and what to expect.
If I don't trust the PM, I just add massive paddings.
You know in your mind it will take you 3 days
You give me, your PM, an estimate of 2 weeks
I report up the chain that it will take a month
When it's all said and done, you end up getting it done in a month by the skin of your teeth :-)
I had PMs to who I could give optimistic estimations and risks. That way, the PM could plan the different scenarios (like extra features in case we had spare time).
Other PMs, the ones who converted estimations into deadlines, got lots of padding. And the problem of adding padding is that it is time lost, because a developer who adds padding cannot tell that they finished earlier.