Update January '23: 10 years later, a refinement on this thought.
I am bad at planning how long something will take. Really quite horrible at it. Based on results from the past 12 months, I can safely say my estimates actually take 2-4 times longer. I guess the bright side is that I can now predict more accurately: I need to ultimately multiply timeframes by 4. Not bad, if you look at it that way... A lot more predictable!
Perhaps the reason I'm bad at this (unless you assume it's down to not having a decade of experience in professional time estimates) is that figuring out how long something will take is usually associated with deadlines. I don't like deadlines; no – I think they're a relic of an older system.
If someone has given you a deadline, then there isn't all that much you can do if you actually need to get the job done. You could try specifying upfront that it will be ready when it's ready, but if you do that then there's a very high chance you won't get the job in the first place. Tough.
If, on the other hand, you're setting a deadline for yourself, then that's a different story. Unless there are specific external circumstances that dictate that something needs to be done by a specific time and date, I say you're better off not having the deadline. Here's why.
You're arbitrarily limiting your creative process. You might get your creative breakthrough right as the deadline is approaching (or maybe once it has passed), and by then it is already too late.
You're creating a needlessly stressful situation. As time to finish approaches, you will probably start rushing and feel that you don't have enough time. Not a great feeling, and mistakes can be made. Even the word "deadline" is utterly negative.
Team cohesion can suffer. When everyone is on edge and expecting something from someone else, all sorts of emotions and needless remarks surface as soon as the smallest of details isn't right.
The time set to finish makes no sense. Not only is it not logical (no external circumstances dictated it), but it is also not intuitive. From my experience, I've seen that an intuitive choice is an overwhelmingly better precursor to success (in its various manifestations). So when a deadline is neither intuitively chosen nor logically deduced, it's utterly pointless.
Regarding intuition, sometimes you do things on a whim. Your gut says it's the right thing to do, or life points you in the right direction. This process has never let me down, but this process is also outside our time-based mindset. You can't control when this kicks in, how long it will last, or when it will stop. Deadlines and timeframes are not only meaningless for this profoundly important process, but also harmful.
Having said all this, not having deadlines is not an invitation to procrastinate. There's a difference between patiently and intently waiting, and just wasting time simply because 'there's no deadline.' The approach needs to be more responsible and attentive.