Pawel caught me being ambiguous. See his comment, “1. I've seen features/fixes which required 2 days to be developed and released.”
Sorry, me too. But what I tried to say was this: A feature was estimated to be some duration of person-hours. Those person-hours have come and gone. The feature still requires another 10-12 person-hours, according to the developer. In that case, I don't trust the estimate of 10-12 person-hours left. In my experience, the reasons for the delay generally have not been addressed. If interruptions caused the delay, more interruptions could still happen. If the delay was caused by having to change the architecture, that could happen again (and I suspect it's likely). In my experience, until the feature is done, it's quite difficult to predict when it will be done, which is why I like to start a new iteration to make sure it gets done.
I tried to imagine what would convince me that an updated estimate was more accurate. In the past I've only had one thing that would convince me: I would have to see visible progress of more work complete (more pieces of the feature complete) to agree that the estimate of what's remaining to finish is correct. I'm trying to think of more things, and can't right now.
Labels: estimation, iteration, timebox