Are you someone who goes the extra mile, gives 110%, burns the midnight oil, etc? It turns out that you're just wasting your time. A study out of California says don't bother, no one cares.
Let's say that you promise to have a project done in 3 days. The study says take the full three days. While taking 4 days to complete the project might have the bosses upset, finishing in just two will not impress anyone. A behavioral sciences professor at UC San Diego found that exceeding your promise "does not appear to be worth the effort".
Since no one cares that you're really good at your job, I'd take the "I'll get it done when I get it done" approach. I prefer not to set expectations. This way you're not disappointed!