Depends on 2 things - the app and your experience.
"Building" one of the example apps in the SDK? 5 minutes.
Building your own app, starting with an idea and no programming knowledge whatsoever? Depends on how much time you can devote to it (full time? All weekend every weekend?) and how fast a learner you are but, unless you're talking about some trivial app, a couple of years at least.
("Building an app" is writing a computer program. Granted, things are easier today than when we had to learn the series of ones and zeros that formed the instructions inside the chip, but it's still not something you're going to learn on a rainy afternoon. I've only been "building apps" [not on Androids] for 40 years, and I'd hardly consider my programming education finished. It's barely started. If no one came up with a single new thing in programming for the next thousand years, I'd probably know almost all of it by the end of this century. And someone's coming up with something new every few months. You can't even run fast enough to fall behind any more.)
Edit:
Didn't notice this was in the Intro section (I was reading unanswered posts). Didn't mean to be rude, so please accept my welcome also.
