I think it's very sector dependent.

Working on drivers, a relatively recent example is when we started looking at a "small" image corruption issue in some really specific cases, that slowly spidered out to what was fundamentally a hardware bug affecting an entire class of possible situations, it was just this one case happened to be noticed first.

There was even talk about a hardware ECO at points during this, though an acceptable workaround was eventually found.

I could never have predicted that when I started working on it, and it seemed every time we thought we'd got a decent idea about what was happening even more was revealed.

And then there's been many other issues when you fall onto the cause pretty much instantly and a trivial fix can be completed and in testing faster than updating the bugtracker with an estimate.

True there's probably a decent amount, maybe even 50%, where you can probably have a decent guess after putting in some length of time and be correct within a factor of 2 or so, but I always felt the "long tail" was large enough to make that pretty damn inaccurate.