How long does it take to learn how to develop an Android app?

This is a very generic question, so you're going to get a relatively generic answer: it depends.

1. Do you have any prior programming knowledge? If so, are you well-versed in Java?  If the answer to both of these questions is yes, then it shouldn't take you very long to get up to speed on Android development. If the answer is no, then you're going to want to start by learning programming basics. Learn to code here.
2. Do you have any backend/database knowledge? The backend for a messaging app can be relatively straightforward or very complex, depending on the nature of the messages. Can they be sent to a group, or just person-to-person? Can they be edited, updated, deleted? Can they be flagged as spam? There are lots of things to consider.
3. Do you know graphic design / UX design?  Creating an app is much more than just learning how to code. If you're building the app as an internal tool or just for fun, maybe this piece is less important to you. However, if you're building it as a business, to make a profit, then this is extremely, unforgivingly important. With over 700,000 apps on the app store now, apps have to be well-designed to stand above the noise.

Just from my personal experience, I was well-versed in Ruby on Rails, PHP, and PostgreSQL when I decided I wanted to learn to make my first app from scratch.  I'd say it took me two month to get a good grip on mobile programming, and six months to actually consider myself a good mobile developer. You have to think about things differently on mobile, in terms of functionality, interaction, speed, and memory issues.

So I'd say, if you want to create an app, learning to do so can take anywhere from 2 months to a year.

Hope this helps!

Answered by Scott Lieberman on Quora.com