In my experience people demand fantasies, and will fight tooth and nail any encroachment on them by the reality that things never happen as they are planned. Although when I say this, I am thinking of one year and above kind of estimates.

When people who are experts on the topic evaluate the work needed over a period of say 3 months, even in something as notoriously hard to plan as video game production, it can hold. This entails being willing to adjust scope and resources though, when planning, in order to ensure the objectives are likely to be met.