I work at Snapchat so I might be qualified to answer this.

To better understand how long it will take you to build an app like Snapchat, you need to know what skills you will need.

Now if we assume that you have at least a moderate level of understanding of everything I have mentioned above, it is safe to guess that it will take you at least 3–6 months of full-time work to actually code an app with the basic functionality of Snapchat.

P.S. There are a lot of advanced areas that I did not mention:

  1. Snapchat uses encryption for their Snaps.
  2. Machine learning is used heavily throughout the app.
  3. A great deal of work has been put to secure the servers.
  4. Some camera features involve AR.
  5. and so on..