An appetite is completely different from an estimate. Estimates start with a design and end with a number. Appetites start with a number and end with a design. We use the appetite as a creative constraint on the design process.
This principle, called “fixed time, variable scope,” is key to successfully defining and shipping projects. […]
We apply this principle at each stage of the process, from shaping potential projects to building and shipping them. First, the appetite constrains what kind of a solution we design during the shaping process. Later, when we hand the work to a team, the fixed time box pushes them to make decisions about what is core to the project and what is peripheral or unnecessary.
-Ryan Singer, Shape Up: Stop Running in Circles and Ship Work that Matters (chapter 3)
In other words, the most effective way to achieve the desired level of quality is to narrow the scope rather than increasing the amount of time dedicated to the project. You can always follow up later with another project that extends the scope, if doing so is still important enough.