How often do you know that that far ahead? If someone asked me to guarantee that a service will be replaced in 4 years that'd be hard. Ask me about 12 years and it's basically impossible.
if it's a project for your employer that you have staff in charge of maintaining, then it's probably a bad choice. but for projects delivered by contractors who provide a fixed service and maintenance term, it's reasonable to provide a fixed end date for the project as a whole, and that's probably less than 12 years.