This is an almost impossible question to answer without knowing any more details. If you wanted to build a really small app for private use? You might be able to do it in a single day. A medium sized app for a company with high complexity and a lot of compliancy requirements? Might take you months to years, depending on requirements.
You could look at some of the blog posts / use cases or Mendix world sessions that are similar to the application you're thinking of, and seeing how long it took them. That could at least give you somewhat of an indication.